| By Author | [ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | Other Symbols ] |
| By Title | [ A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | Other Symbols ] |
| By Language |
|
Download this book: [ ASCII ] Look for this book on Amazon Tweet |
Title: The Story of a Boulder; or, Gleanings from the Note-book of a Field Geologist
Author: Geikie, Sir Archibald
Language: English
As this book started as an ASCII text book there are no pictures available.
*** Start of this LibraryBlog Digital Book "The Story of a Boulder; or, Gleanings from the Note-book of a Field Geologist" ***
This book is indexed by ISYS Web Indexing system to allow the reader find any word or number within the document.
GLEANINGS FROM THE NOTE-BOOK OF A FIELD GEOLOGIST ***
THE STORY OF A BOULDER.
THE STORY OF A BOULDER
GLEANINGS FROM THE NOTE-BOOK OF A FIELD GEOLOGIST
BY
ARCHIBALD GEIKIE
OF THE GEOLOGICAL SURVEY OF GREAT BRITAIN.
Illustrated with Woodcuts.
EDINBURGH: THOMAS CONSTABLE AND CO. HAMILTON, ADAMS, AND CO., LONDON.
MDCCCLVIII.
EDINBURGH: T. CONSTABLE, PRINTER TO HER MAJESTY.
TO
GEORGE WILSON, M.D., F.R.S.E.
REGIUS PROFESSOR OF TECHNOLOGY IN THE UNIVERSITY OF EDINBURGH,
THESE PAGES
ARE
AFFECTIONATELY INSCRIBED.
PREFACE
The present Volume has been written among the rocks which it seeks
to describe, during the intervals of leisure of a field-geologist.
Its composition has been carried on by snatches, often short and far
apart, some of the descriptions having been jotted down on the spot
by streamlet and hill-side, or in the quiet of old quarries; others,
again, in railway-carriage or stage-coach. By much the larger portion,
however, has been written by the village fireside, after the field-work
of the day was over--a season not the most favourable to any mental
exercise, for weariness of body is apt to beget lassitude of mind. In
short, were I to say that these Chapters have been as often thrown
aside and resumed again as they contain paragraphs, the statement would
probably not exceed the truth. But the erratic life of an itinerant
student of science is attended with yet greater disadvantages. It
entails an absence from all libraries, more especially scientific
ones, and the number of works of reference admissible into his _parva
supellex_ must ever be few indeed. With these hindrances, can the
writer venture to hope that what has thus been so disjointed and
unconnected to him, will not seem equally so to his readers? Yet if his
descriptions, written, as it were, face to face with Nature, are found
to have caught some tinge of Nature's freshness, and please the reader
well enough to set him in the way of becoming a geologist, he shall
have accomplished all his design.
It cannot be too widely known, or too often pressed on the attention,
especially of the young, that a true acquaintance with science, so
delightful to its possessors, is not to be acquired at second-hand.
Text-books and manuals are valuable only so far as they supplement
and direct our own observations. A man whose knowledge of Nature is
derived solely from these sources, differs about as much from one who
betakes himself to Nature herself, as a dusty, desiccated mummy does
from a living man. You have the same bones and sinews in both; but
in the one they are hard and dry, wholly incapable of action; in the
other they are instinct with freshness and life. He who would know what
physical science really is, must go out into the fields and learn it
for himself: and whatever branch he may choose, he will not be long in
discovering that a forenoon intelligently spent there must be deemed
of far more worth than days and weeks passed among books. He sees the
objects of his study with his own eyes, and not through "the spectacles
of books;" facts come home to him with a vividness and reality they
never can possess in the closet; the free buoyant air brightens his
spirits and invigorates his mind, and he returns again to his desk or
his workshop with a store of new health and pleasure and knowledge.
Geology is peculiarly rich in these advantages, and lies in a manner
open to all. No matter what may be the season of the year, it offers
always some material for observation. In the depth of winter we have
the effects of ice and frost to fall back upon, though the country
should lie buried in snow; and then when the longer and brighter
days of spring and summer come round, how easily may the hammer be
buckled round the waist, and the student emerge from the dust of town
into the joyous air of the country, for a few delightful hours among
the rocks; or when autumn returns with its long anticipated holidays,
and preparations are made for a scamper in some distant locality,
hammer and note-book will not occupy much room in the portmanteau,
and will certainly be found most entertaining company. The following
pages--forming a digest of the Carboniferous rocks--may, perhaps, in
some measure, guide the explorations of the observer, by indicating
to him the scope of geological research, the principles on which the
science rests, and the mode in which it is pursued. But I repeat, no
book, no lecture-room, no museum, will make a geologist of him. He must
away to the fields and study for himself, and the more he can learn
there he will become the better geologist.
He need not burden himself with accoutrements. A hammer, pretty stout
in its dimensions, with a round blunt face and a flat sharp tail; a
note-book and a good pocket-lens, are all he needs to begin with.
Having these, let him seek to learn the general characters' of the
more common rocks, aiding himself, where he can, by a comparison with
the specimens of a museum, or, failing that, with the descriptions of
a text-book. Let him then endeavour to become acquainted with some of
the more characteristic fossils of the district in which he resides,
so as to be able to recognise them wherever they occur. Private
collections and local museums are now becoming comparatively common,
and these, where accessible, will aid him vastly in his studies. Having
at length mastered the more abundant rocks and organic remains of his
neighbourhood, let him try to trace out the connexion of the different
strata across the country, so as to understand its structure. For
this purpose it will be necessary to examine every ravine and natural
exposure of the rocks, along with quarries, ditches, railway-cuttings,
and, in short, the whole surface of the district. A general notion
of the geology of the place will, not perhaps be of very difficult
attainment; and this done, the observer should attempt to put down
the connexion of the rocks on paper, for till this is accomplished he
will have at the best but an imperfect, and perhaps incorrect notion
of the subject. The best map of the district should be obtained, also
a clinometer, or instrument for ascertaining the angle at which rocks
_dip_ with the horizon, and a pocket-compass with which to mark the
direction of the _dip_ and _strike_ of strata, that is, the _outcrop_,
or line which they form when they come to the surface. Thus armed, he
may commence a geological survey of his neighbourhood. Wherever he
sees a bed of rock exposed, it should be marked down on his map with
an arrow pointing to the direction in which the stratum is dipping,
the angle of dip, ascertained by the clinometer, being put alongside.
The nature of the rock, whether sandstone, shale, limestone, or
greenstone, must be set down at the same place, and, to save room,
a system of marks for the different rocks may be conveniently used.
When a sufficient area of ground has been thus traversed, the student
may find, say a row of arrows on his map all pointing due west, and
indicating a set of quarries about a quarter of a mile distant from one
another, the rock in each of them dipping to the west. If there be at
the one end a limestone containing certain fossils, and at the other
end a stratum exactly similar, containing the same fossils, while the
quarries between display the same rock, he will infer, of course, that
the whole is one limestone, and will accordingly draw a line from the
last quarry on the north to the last on the south, connecting them all
together. If the bed dips steeply down, the line will be narrower,--if
but slightly inclined, it will be broader; the breadth of such a line
(which may be coloured to taste) always varying with the thickness
of the stratum and the angle which it makes with the horizon. In a
district where faults and curvatures along with trap-rocks abound, the
mapping becomes more complex, but the principle remains the same--a
curved stratum on the ground making a similarly curved band on the map,
and a fault or dislocation of a set of beds producing, in the same way,
a corresponding break in the lines traced. In short, a geological map
should be as far as possible a transcript of the surface rocks of a
country. The beginner should avoid, however, attempting too much; it
will be enough for him at first to have mastered the leading features
of the geology of his district; the details cannot be shown save on
a map of a large scale, and are better transferred to his note-book.
The use of such mapping is to enable us to gain a correct knowledge of
the geological structure of a country, and of the relation of rocks to
each other as regards age, origin, &c. Bacon tells us that "writing
makes an exact man;" we may say with equal truth that mapping makes
an exact geologist. It is sometimes easy enough to obtain a notion of
the general character of a district by taking a few rambles across it;
but we can never know it thoroughly until we have mapped it. And this
is done not as mere dry routine, or by a series of hard uninteresting
rules. In reading off the geological structure of a country, we
ascertain its history during many thousand ages long prior to that
of man. We become, as it were, interpreters of hieroglyphics, and
historians of long-perished dynasties.
Those who have had experience of field-geology, know how vain it is
to attempt to compress into a page or two the results of years, and
that a few vague general directions are about the utmost that can be
attempted. The practice of the science cannot be taught in books, far
less in prefaces, neither can it be learned from them. And so I once
more repeat the advice: Get away to the fields. Seek to decipher the
geological records for yourself, and look with your own eyes into the
long series of ages whose annals lie inscribed among the rocks. If you
can secure the co-operation of a few companions, so much the better.
Half-a-dozen hammers zealously at work in a richly fossiliferous
stratum will soon pile up a tolerable collection of its treasures. But
whether singly or in company, use your eyes and your hammer, and even
though in the end you should never become a geologist, you will in the
meantime gain health and vigour, and a clearness of observation, that
will stand you in good stead through life.
CONTENTS.
CHAPTER I.
PAGE
Scene near Colinton in midsummer--A grey travelled Boulder--Its
aspect and contents--Its story of the past,
1
CHAPTER II.
Exterior of the boulder--Travelled stones a difficult problem--Once
referred to the Deluge--Other theories--Novelty of the true
solution--Icebergs formed in three ways--Progress and scenery
of an iceberg--Its effects--Size of icebergs--Boulder clay
had a glacial origin--This explanation confirmed by fossil
shells--Laws of the distribution of life--Deductions,
6
CHAPTER III.
How the boulder came to be one--"Crag and tail"--Scenery of
central Scotland: Edinburgh--"Crag and tail" formerly
associated in its origin with the boulder-clay--This
explanation erroneous--Denudation an old process--Its
results--Illustration from the Mid-Lothian coal-field--The
three Ross-shire hills--The Hebrides relics of an
ancient land--Scenery of the western coast--Effects of
the breakers--Denudation of the Secondary strata of the
Hebrides--Preservative influence of trap-rocks--Lost
species of the Hebrides--Illustration--Origin of the
general denudation of the country--Illustrative action
of streams--Denudation a very slow process--Many old
land-surfaces may have been effaced--Varied aspect of the
British Islands during a period of submergence--Illustration,
18
CHAPTER IV.
Interior of the boulder--Wide intervals of
Geology--Illustration--Long interval between the
formation of the boulder as part of a sand-bed,
and its striation by glacial action--Sketch of the
intervening ages--The boulder a Lower Carboniferous
rock--Cycles of the astronomer and the geologist
contrasted--Illustration--Plants shown by the boulder once
grew green on land--Traces of that ancient land Its seas,
shores, forests, and lakes, all productive of material
aids to our comfort and power--Plants of the Carboniferous
era--Ferns--Tree-ferns--Calamites--Asterophyllites--
Lepidodendron--Lepidostrobus--Stigmaria--Scene in a ruined
palace--Sigillaria--Coniferæ, Cycadeæ--Antholites, the oldest
known flower--Grade of the Carboniferous flora--Its resemblance
to that of New Zealand,
30
CHAPTER V.
Scenery of the carboniferous forests--Contrast in the
appearance of coal districts at the present day--Abundance
of animal life in the Carboniferous era--Advantages
of palæontology over fossil-botany--Carboniferous
fauna--Actiniæ--Cup-corals--Architecture of the present
day might be improved by study of the architecture of the
Carboniferous period--Mode of propagation of corals--A
forenoon on the beach--Various stages in the decomposition of
shells--Sea-mat--Bryozoa--Fenestella--Retepora--Stone-lilies--
Popular superstitions--Structure of the stone-lilies--Aspect
of the sea-bottom on which the stone-lilies
flourished--Sea-urchins--Crustacea, their high
antiquity--Cyprides--Architecture of the Crustacea and
mollusca contrasted--King-crabs,
59
CHAPTER VI.
Carboniferous fauna continued--George Herbert's ode on
"Man"--His idea of creation--What nature teaches on
this subject--Molluscous animals--Range of species in
time proportionate to their distribution in space--Two
principles of renovation and decay exhibited alike in the
physical world and the world of life--Their effects--The
mollusca--Abundantly represented in the carboniferous
rocks--Pteropods--Brachiopods--Productus--Its alliance with
Spirifer--Spirifer--Terebratula--Lamellibranchs--Gastropods--
Land-snail of Nova Scotia--Cephalopods--Structure of
orthoceras--Habits of living nautilus,
86
CHAPTER VII.
Classification of the naturalist not always correspondent with
the order of nature--Incongruous grouping of animals
in the invertebrate division--Rudimentary skeleton
of the cephalopods--Introduction of the vertebrate
type into creation--Ichthyolites of the carboniferous
rocks--Their state of keeping--Classification of fossil
fishes--Placoids--Ichthyodorulites--Ganoids--Their structure
exemplified in the megalichthys and holoptychius--Cranium of
megalichthys--Its armature of scales--Microscopic structure of
a scale--Skeleton of megalichthys--History of the discovery
of the holoptychius--Confounded with megalichthys--External
ornament of holoptychius--Its jaws and teeth--Microscopic
structure of the teeth--Paucity of terrestrial fauna in coal
measures--Insect remains--Relics of reptiles--Concluding
summary of the characters of the Carboniferous fauna--Results,
110
CHAPTER VIII.
Sand and gravel of the boulder--What they suggested--Their
consideration leads us among the more mechanical operations
of Nature--An endless succession of mutations in the
economy of the universe--Exhibited in plants In animals--In
the action of winds and oceanic currents--Beautifully
shown by the ceaseless passage of water from land to
sea, and sea to land--This interchange not an isolated
phenomenon--How aided in its effects by a universal process
of decay going on wherever a land surface is exposed to
the air--Complex mode of Nature's operations--Interlacing
of different causes in the production of an apparently
single and simple effect--Decay of rocks--Chemical
changes--Underground and surface decomposition--Carbonated
springs--The Spar Cave--Action of rain-water--Decay of
granite--Scene in Skye--Trap-dykes--Weathered cliffs of
sandstone--Of conglomerate--Of shale--Of limestone--Caverns
of Raasay--Incident--Causes of this waste of calcareous
rocks--Tombstones,
138
CHAPTER IX.
Mechanical forces at work in the disintegration of
rocks--Rains--Landslips--Effects of frosts--Glaciers and
icebergs--Abrading power of rivers--Suggested volume on the
geology of rivers--Some of its probable contents--Scene
in a woody ravine--First idea of the origin of the ravine
one of primeval cataclysms--Proved to be incorrect--Love
of the marvellous long the bane of geology--More careful
examination shows the operations of Nature to be singularly
uniform and gradual--The doctrine of slow and gradual change
not less poetic than that of sudden paroxysms--The origin
of the ravine may be sought among some of the quieter
processes of Nature--Features of the ravine Lessons of the
waterfall--Course of the stream through level ground--True
history of the ravine--Waves and currents--What becomes of the
waste of the land--The Rhone and the Leman Lake--Deltas on the
sea-margin--Reproductive effects of currents and waves--Usual
belief in the stability of the land and the mutability of the
ocean--The reverse true--Continual interchange of land and
sea part of the economy of Nature--The continuance of such a
condition of things in future ages rendered probable by its
continuance during the past,
157
CHAPTER X.
The structure of the stratified part of the earth's
crust conveniently studied by the examination of a
single formation--A coal-field selected for this
purpose--Illustration of the principles necessary to such
an investigation--The antiquities of a country of value in
compiling its pre-historic annals--Geological antiquities
equally valuable and more satisfactorily arranged--Order
of superposition of stratified formations--Each formation
contains its own suite of organic remains--The age of
the boulder defined by this test from fossils--Each
formation as a rule shades into the adjacent ones--Mineral
substances chiefly composing the stratified rocks few
in number--Not of much value in themselves as a test of
age--The Mid-Lothian coal-basin--Its subdivisions--The
limestone of Burdiehouse--Its fossil remains--Its probable
origin--Carboniferous limestone series of Mid-Lothian--Its
relation to that of England--Its organic remains totally
different from those of Burdiehouse--Structure and
scenery of Roman Camp Hill--Its quarries of the mountain
limestone--Fossils of these quarries indicative of an ancient
ocean-bed--Origin of the limestones--Similar formations still
in progress--Coral-reefs and their calcareous silt--Sunset
among the old quarries of Roman Camp Hill,
178
CHAPTER XI.
Intercalation of coal seams among mountain limestone beds of
Mid-Lothian--North Greens seam--Most of our coal seams
indicate former land-surfaces--Origin of coal a debated
question--Erect fossil trees in coal-measures--Deductions to
be drawn therefrom--Difference between the mountain limestone
of Scotland and that of England--Coal-bearing character of the
northern series--Divisions of the Mid-Lothian coal-field--The
Edge coals--Their origin illustrated by the growth of modern
deltas--Delta of the Nile--Of the Mississippi--Of the
Ganges--Progress of formation of the Edge coals--Scenery
of the period like that of modern deltas--Calculations of
the time required for the growth of a coal-field--Why of
doubtful value--Roslyn Sandstone group--Affords proofs of
a general and more rapid subsidence beneath the sea--Its
great continuity--Probable origin--Flat coals--Similar
in origin to the Edge coals below--Their series not now
complete--Recapitulation of the general changes indicated by
the Mid-Lothian coal-field,
204
CHAPTER XII.
Trap-pebbles of the boulder--Thickness of the earth's crust
unknown--Not of much consequence to the practical
geologist--Interior of the earth in a highly heated
condition--Proofs of this--Granite and hypogene
rocks--Trap-rocks: their identity with lavas and
ashes--Scenery of a trappean country--Subdivisions of
the trap-rocks--Intrusive traps--Trap-dykes--Intrusive
sheets--Salisbury Crags--Traps of the neighbourhood of
Edinburgh--Amorphous masses--Contemporaneous trap-rocks of
two kinds--Contemporaneous melted rocks--Tests for their age
and origin--Examples from neighbourhood of Edinburgh--Tufas
or volcanic ashes--Their structure and origin--Example
of contemporaneous trap-rocks--Mode of interpreting
them--Volcanoes of Carboniferous times--Conclusion,
235
THE STORY OF A BOULDER.
CHAPTER I.
Scene near Colinton in midsummer--A grey travelled Boulder--Its
aspect and contents--Its story of the past.
Three miles to the south-west of Edinburgh, and not many hundred yards
from the sequestered village of Colinton, there is a ravine, overshaded
by a thick growth of beech and elm, and traversed beneath by a stream,
which, rising far away among the southern hills, winds through the
rich champaign country of Mid-Lothian. It is, at all seasons of the
year, one of the most picturesque nooks in the county. I have seen it
in the depth of winter--the leafless boughs doddered and dripping, the
rocks dank and bare save where half-hidden by the rotting herbage, and
the stream, red and swollen, roaring angrily down the glen, while the
families, located along its banks, fleeing in terror to the higher
grounds, had left their cottages to the mercy of the torrent. The last
time I visited the place was in the heart of June, and surely never did
woodland scene appear more exquisitely beautiful. The beech trees were
in full leaf, and shot their silvery boughs in slender arches athwart
the dell, intertwining with the broader foliage and deeper green of
the elm, and the still darker spray of the stately fir. The rocks on
either side were tapestried with verdure; festoons of ivy, with here
and there a thread of honey-suckle interwoven, hung gracefully from the
cliffs overhead; each projecting ledge had its tuft of harebells, or
speedwell, or dog-violets, with their blue flowers peeping out of the
moss and lichens; the herb-robert trailed its red blossoms over crag
and stone; the wood-sorrel nestled its bright leaves and pale flowerets
among the gnarled roots of beech and elm; while high over all, alike on
the rocks above and among the ferns below, towered the gently drooping
stalks of the fox-glove. The stream, almost gone, scarcely broke the
stillness with a low drowsy murmur, as it sauntered on among the
_lapides adesos_ of its pebbly channel. Horace's beautiful lines found
again their realization:--
"Qua pinus ingens, albaque populus
Umbram hospitalem consociare amant
Ramis, et obliquo laborat
Lympha fugax trepidare rivo." [1]
[Footnote 1:
Where the tall pine and poplar pale
Delight to cast athwart the vale
A pleasing shade.
While the clear stream low murmuring bells.
And o'er its winding channel toils
Adown the glade.--A. G.
]
It was noon, and the sun shone more brightly and with greater heat
than had been felt for years. The air, heavy and warm, induced a
feeling of listlessness and languor, and the day seemed one for which
the only appropriate employment would have been to read once again
the "Castle of Indolence." But failing that, I found it pleasant to
watch the flickering light shot in fitful gleams through the thick
canopy of leaves, and thus, in the coolness of the shade, to mark
these rays--sole messengers from the sweltering world around--as they
danced from rock to stream, now lighting up the ripples that curled
dreamily on, now chequering some huge boulder that lay smooth and
polished in mid-channel, anon glancing playfully among the thickets
of briar or honeysuckle and vanishing in the shade. Sometimes a
wagtail would alight at hand, or a bee drone lazily past, while even
an occasional butterfly would venture down into this shady covert.
But, with these exceptions, the animal creation seemed to have gone to
sleep, an example which it was somewhat difficult to avoid following.
While thus idly engaged, my eye rested on a large boulder on the
opposite side. It lay partly imbedded in a stiff clay, and partly
protruding from the surface of the bank some way above the stream.
A thick arbour of leafage overhung it, through which not even the
faintest ray of sunshine could force its way. The spot seemed cooler
and more picturesque than that which I occupied, and so, crossing the
well-nigh empty channel, I climbed the bank and was soon seated on the
boulder. A stout hammer is a constant companion in my rambles, and was
soon employed on this occasion in chipping almost unconsciously the
newly-acquired seat. The action was, perhaps, deserving of the satire
of Wordsworth's Solitary:--
"You may trace him oft
By scars, which his activity has left
Beside our roads and pathways, though, thank Heaven!
This covert nook reports not of his hand.
He, who with pocket-hammer smites the edge
Of luckless rock or prominent stone, disguised
In weather-stains, or crusted o'er by Nature
With her first growths, detaching by the stroke
A chip or splinter to resolve his doubts;
And, with that ready answer satisfied,
The substance classes by some barbarous name,
And hurries on; or from the fragments picks
His specimen; if but imply interveined
With sparkling mineral, or should crystal cube
Lurk in its cells and thinks himself enriched.
Wealthier, and doubtless wiser, than before!"
There was nothing in the distant aspect of the boulder to attract
attention. It was just such a mass as dozens of others all round. Nor,
on closer inspection, might anything peculiar have been observed. It
had an irregularly oblong form, about two or three feet long, and half
as high. Ferns and herbage were grouped around it, the wood-sorrel
clustered up its sides, and little patches of moss and lichen nestled
in its crevices. And yet, withal, there was something about it that,
ere long, riveted my attention. I examined it minutely from one end to
the other, and from top to bottom. The more I looked the more did I see
to interest me; and when, after a little labour, some portions of its
upper surface were detached, my curiosity was abundantly gratified.
That grey lichened stone, half hid among foliage, and unheeded by any
human being, afforded me material for a pleasant forenoon's thought.
Will my reader accept an expanded narrative of my reverie?
I can almost anticipate a smile. "What can there be remarkable in such
a grey stone, hidden in a wood, and of which nobody knows anything?
It never formed part of any ancient building; it marks the site of
no event in the olden time; it is linked with nothing in the history
of our country. What of interest, then, can it have for us?" Nay, I
reply, you are therein mistaken. It is, assuredly, linked with the
history of our country--it does mark the passing of many a historical
event long ere human history began; and, though no tool ever came upon
it, it did once form part of a building that rose under the finger of
the Almighty during the long ages of a bygone eternity. To change the
figure, this boulder seemed like a curious volume, regularly paged,
with a few extracts from older works. Bacon tells us that "some books
are to be tasted, others to be swallowed, and some few to be chewed and
digested." Of the last honour I think the boulder fully worthy, and if
the reader will accompany me, I shall endeavour to show him how the
process was attempted by me.
The rock consisted of a hard grey sandstone finely laminated above,
and getting pebbly and conglomeritic below. The included pebbles were
well worn, and belonged to various kinds of rock. The upper part of
the block was all rounded, smoothed, and deeply grooved, and, when
split open, displayed numerous stems and leaflets of plants converted
into a black coaly substance. These plants were easily recognisable
as well-known organisms of the carboniferous strata, and it became
accordingly evident that the boulder was a block of carboniferous
sandstone. The pebbles below, however, must have been derived from
more ancient rocks, and they were thus seen to represent some older
geological formation. In this grey rock, therefore, there could at
once be detected well-marked traces of at least two widely-separated
ages. The evidence for each was indubitable, and the chronology of
the whole mass could not be mistaken. The surface striation bore
undoubted evidence of the glacial period, the embedded plants as
plainly indicated the far more ancient era of the coal-measures, while
the pebbles of the base pointed, though dimly, to some still more
primeval age. I had here, as it were, a quaint, old, black-letter
volume of the middle ages, giving an account of events that were
taking place at the time it was written, and containing on its earlier
pages numerous quotations from authors of antiquity. The scratched
surface, to complete the simile, may be compared to this old work
done up in a modern binding. Let us, then, first of all, look for a
little at the exterior of the volume, and inquire into the origin of
that strangely-striated surface, and of the clay in which the boulder
rested.
CHAPTER II.
Exterior of the boulder--Travelled stones a difficult problem--Once
referred to the Deluge--Other theories--Novelty of the true
solution--Icebergs formed in three ways--Progress and scenery
of an iceberg--Its effects--Size of icebergs--Boulder-clay
had a glacial origin--This explanation confirmed by fossil
shells--Laws of the distribution of life--Deductions.
Has the reader, when wandering up the course of a stream, rod in hand
perhaps, ever paused at some huge rounded block of gneiss or granite
damming up the channel, and puzzled himself for a moment to conjecture
how it could get there? Or when rolling along in a railway carriage,
through some deep cutting of sand, clay, and gravel, did the question
ever obtrude itself how such masses of water-worn material came into
existence? Did he ever wonder at the odd position of some huge grey
boulder, far away among the hills, arrested as it were on the steep
slope of a deep glen, or perched on the edge of a precipitous cliff, as
though a push with the hand would hurl it down into the ravine below?
Or did he ever watch the operations of the quarryman, and mark, as each
spadeful of soil was removed, how the surface of the rock below was all
smoothed, and striated, and grooved?
These questions, seemingly simple enough, involve what was wont to
be one of the greatest problems of geology, and not many years have
elapsed since it was solved. The whole surface of the country was
observed to be thickly covered with a series of clays, gravels, and
sands, often abounding in rounded masses of rock of all sizes up to
several yards in diameter. These deposits were seen to cover all
the harder rocks, and to occur in a very irregular manner, sometimes
heaped up into great mounds, and sometimes entirely wanting. They were
evidently the results of no agency visible now, either on the land
or around our coasts. They had an appearance rather of tumultuous
and violent action, and so it was wisely concluded that they must be
traces of the great deluge. The decision had at least this much in its
favour, it was thoroughly orthodox, and accordingly received marked
approbation, more especially from those who wished well to the young
science of geology, but were not altogether sure of its tendencies.
But, alas! this promising symptom very soon vanished. As observers
multiplied, and investigations were carried on in different countries,
the truth came out that these clays and gravels were peculiarly a
northern formation; that they did not appear to exist in the south of
France, Italy, Asia Minor, Syria, and the contiguous countries. If,
then, they originated from the rushing of the diluvian waters, these
southern lands must have escaped the catastrophe, and the site of the
plains of Eden would have to be sought somewhere between the Alps
and the North Pole. This, of course, shocked all previous ideas of
topography; it was accordingly agreed, at least among more thoughtful
men, that with these clays and sands the deluge could have had nothing
to do.
Other theories speedily sprang up, endeavouring to account for the
phenomena by supposing great bodies of water rushing with terrific
force across whole continents, sweeping away the tops of hills,
tearing up and dispersing entire geological formations, and strewing
the ocean-bottom with scattered debris. But this explanation had the
disadvantage of being woefully unphilosophical and not very clearly
orthodox. Such debacles did not appear to have ever taken place in
any previous geologic era, and experience was against them. Besides,
they did not account for some of the most evident characteristics of
the phenomena, such as the northern character of the formation, the
long parallel striations of the rock surfaces, and the perching of
huge boulders on lofty hills, often hundreds of miles distant from the
parent rock. Geologists were completely at fault, and the boulder-clay
remained a mystery for years.
When we consider the physical aspects of the countries where the
question was studied, we cannot much wonder that the truth was so hard
to find. In the midst of corn-fields and meadows, one cannot readily
realize the fact that the spot where they stand has been the site of
a wide-spread sea; and that where now villages and green lanes meet
the eye, there once swam the porpoise and the whale, or monsters of
a still earlier creation, unwieldy in bulk and uncouth in form. Such
changes, however, must have been, for their traces meet us on every
hand. We have the sea dashing against our shores, and there seems
nothing at all improbable in the assertion that once it dashed against
our hill-tops. No one, therefore, has any difficulty in giving such
statements his implicit belief. But who could have dreamed that these
fields, so warm and sunny, were once sealed in ice, and sunk beneath
a sea that was cumbered with many a wandering iceberg? Who could have
imagined, that down these glens, now carpeted with heath and harebell,
the glacier worked its slow way amid the stillness of perpetual snow?
And yet strange as it may seem, such is the true solution of the
problem. The boulder-clay was formed during the slow submergence of our
country beneath an icy sea, and the rock-surfaces owe their polished
and striated appearance to the grating across them of sand and stones
frozen into the bottom of vast icebergs, that drifted drearily from the
north. That we may the better see how these results have been effected,
let us glance for a little at the phenomena observable in northern
latitudes at the present day.
Icebergs are formed in three principal ways:--1st, By glaciers
descending to the shore, and being borne seawards by land-winds; 2d,
By river-ice packed during spring, when the upper reaches of the rivers
begin to thaw; 3d, By coast-ice.
I. There is an upper stratum of the atmosphere characterized by intense
cold, and called the region of perpetual snow. It covers the earth
like a great arch, the two ends resting, one on the arctic, the other
on the antarctic zone, while the centre, being about 16,000 feet above
the sea,[2] rises directly over the tropics. Wherever a mountain is
sufficiently lofty to pierce this upper stratum, its summit is covered
with snow, and, as the snow never melts, it is plain that, from the
accumulations of fresh snow-drifts, the mountain-tops, by gradually
increasing in height and width, would become the supporting columns
of vast hills of ice, which, breaking up at last from their weight
and width, would roll down the mountain-sides and cover vast areas
of country with a ruin and desolation more terrible than that of any
avalanche. Olympus would really be superposed upon Ossa. By a beautiful
arrangement this undue growth is prevented, so that the hill-tops
never vary much in height above the sea. The cone of ice and snow
which covers the higher part of the mountain, sends down into each of
the diverging valleys a long sluggish stream of ice, with a motion so
slow as to be almost imperceptible. These streams are called glaciers.
As they creep down the ravines and gorges, blocks of rock detached
by the frosts from the cliffs above, fall on the surface of the ice,
and are slowly carried along with it. The bottom also of the glaciers
is charged with sand, gravel, and mud, produced by the slow-crushing
movement; large rocky masses become eventually worn down into
fragments, and the whole surface of the hard rock below is traversed
by long parallel grooves and striæ in the direction of the glacier's
course. Among the Alps, the lowest point to which the glacier descends
is about 8500 feet. There the temperature gets too high to allow of its
further progress, and so it slowly melts away, choking up the valleys
with piles of rocky fragments called moraines, and 'giving rise to
numerous muddy streams that traverse the valleys, uniting at length
into great rivers such as the Rhone, which enters the Lake of Geneva
turbid and discoloured with glacial mud.
[Footnote 2: The average height of the snow-line within the tropics
is 15,207 feet, but it varies according to the amount of land and sea
adjacent, and other causes. Thus, among the Bolivian Andes, owing to
the extensive radiation, and the ascending currents of air from the
neighbouring plains and valleys, the line stands at a level of 18,000
feet, while, on mountains near Quito, that is, immediately on the
equatorial line, the lowest level is 15,795.--See Mrs. Somerville's
_Physical Geography_, 4th edit. p. 314.]
In higher latitudes, where the lower limit of the snow-line descends to
the level of the sea, the glaciers are often seen protruding from the
shore, still laden with blocks that have been carried down from valleys
far in the interior. The action of storms and tides is sufficient to
detach large masses of the ice, which then floats off, and is often
wafted for hundreds of miles into temperate regions, where it gradually
melts away. Such floating islands are known as icebergs.
II. In climates such as that of Canada, where the winters are very
severe, the rivers become solidly frozen over, and, if the frost be
intense enough, a cake of ice forms at the bottom. In this way sand,
mud, and rocky fragments strewing the banks or the channel of the
stream, are firmly enclosed. When spring sets in, and the upper parts
of the rivers begin to thaw, the swollen waters burst their wintry
integuments, and the ice is then said to _pack_. Layer is pushed
over layer, and mass heaped upon mass, until great floes are formed.
These have often the most fantastic shapes, and are borne down by the
current, dropping, as they go, the mud and boulders, with which they
are charged, until they are stranded along some coast line, or melt
away in mid-ocean.
III. But icebergs are also produced by the freezing of the water of
the ocean. In high latitudes, this takes place when the temperature
falls to 28·5° of Fahrenheit. The surface of the sea then parts with its
saline ingredients, and takes the form of a sheet of ice, which, by
the addition of successive layers, augmented sometimes by snow-drifts,
often reaches a height of from thirty to forty feet. On the approach of
summer these ice-fields break up, crashing into fragments with a noise
like the thundering of cannon. The disparted portions are then carried
towards the equator by currents, and may be encountered by hundreds
floating in open sea. Their first form is flat, but, as they travel on,
they assume every variety of shape and size.
On the shores of brackish seas, such as the Baltic, or along a coast
where the salt water is freshened by streams or snow-drifts from the
land, sheets of ice also frequently form during severe frosts. Sand and
boulders are thus frozen in, especially where a layer of ice has formed
upon the sea-bottom.[3] The action of gales or of tides is sufficient
to break up these masses, which are then either driven ashore and
frozen in a fresh cake of ice, or blown away to sea. The bergs formed
in this way have originally a low flat outline, and many extend as
ice-fields over an area of many miles, while, at a later time, they may
be seen towering precipitously as great hills, some 200 or 300 feet
high.
[Footnote 3: I was informed by the late Mr. Hugh Miller, that a seam
of shale abounding in liassic fossils, had been found intercalated
among the boulder-clay beds in the vicinity of Eathie. He explained its
occurrence there by supposing that it had formed a reef along a shore
where ground-ice was forming; and so having been firmly frozen in, it
was torn up on the breaking of the ice, and deposited at a distance
among the mud at the sea-bottom.]
Few sights in nature are more imposing than that of the huge,
solitary iceberg, as, regardless alike of wind and tide, it steers
its course across the face of the deep far away from land. Like one
of the "Hrim-thursar," or Frost-giants of Scandinavian mythology,[4]
it issues from the portals of the north armed with great blocks of
stone. Proudly it sails on. The waves that dash in foam against its
sides shake not the strength of its crystal walls, nor tarnish the
sheen of its emerald caves. Sleet and snow, storm and tempest, are its
congenial elements. Night falls around, and the stars are reflected
tremulously from a thousand peaks, and from the green depths of
"caverns measureless to man." Dawn again arises, and the slant rays of
the rising sun gleam brightly on every projecting crag and pinnacle,
as the berg still floats steadily on; yet, as it gains more southern
latitudes, what could not be accomplished by the united fury of the
waves, is slowly effected by the mildness of the climate. The floating
island becomes gradually shrouded in mist and spume, streamlets
everywhere trickle down its sides, and great crags ever and anon fall
with a sullen plunge into the deep. The mass becoming top-heavy,
reels over, exposing to light rocky fragments still firmly imbedded.
These, as the ice around them gives way, are dropped one by one into
the ocean, until at last the iceberg itself melts away, the mists are
dispelled, and sunshine once more rests upon the dimpled face of the
deep.[5] If, however, before this final dissipation, the wandering
island should be stranded on some coast, desolation and gloom are
spread over the country for leagues. The sun is obscured, and the air
chilled; the crops will not ripen; and, to avoid the horrors of famine,
the inhabitants are fain to seek some more genial locality until the
ice shall have melted away; and months may elapse before they can
return again to their villages.
[Footnote 4: The account of the origin of these giants, as given in
the prose _Edda_, is very graphic, and may be not inaptly quoted
here:--"When the rivers that are called Elivagar had flowed far from
their sources," replied Har, "the venom which they rolled along;
hardened, as does dross that runs from a furnace, and became ice.
When the rivers flowed no longer, and the ice stood still, the vapour
arising from the venom gathered over it and froze to rime; and in this
manner were formed in Ginnungagap many layers of congealed vapour,
piled one over the other."--"That part of Ginnungagap," added Jafnhar,
"that lies towards the north, was thus filled with heavy masses of
gelid vapour and ice, whilst everywhere within were whirlwinds and
fleeting mists. But the southern part of Ginnungagap was lighted by
the sparks and flakes that flew into it from Muspellheim.... When the
heated blast met the gelid vapour, it melted into drops, and, by the
might of him who sent the heat, these drops quickened into life, and
took a human semblance. The being thus formed was named Ymir, from whom
descend the race of the Frost-giants (Hrim-thursar), as it is said in
the Völuspá, 'From Vidolph came all witches; from Vilmeith all wizards;
from Svarthöfdi all poison-seekers; and all giants from Ymir.'"--See
Mallet's _Northern Antiquities_, edit. Bohn, p. 402.]
[Footnote 5: That beautiful expression of Æschylus occurs to me, so
impossible adequately to clothe in English: ἁνηριθμον γελασμα κυματων.
Who that has spent a calm summer day upon the sea, has not realized its
force and delicate beauty?]
The iceberg melts away, but not without leaving well-marked traces of
its existence. If it disappear in mid-ocean, the mud and boulders, with
which it was charged, are scattered athwart the sea-bottom. Blocks
of stone may thus be carried across profound abysses, and deposited
hundreds of miles from the parent hill; and it should be noticed, that
this is the only way, so far as we know, in which such a thing could
be effected. Great currents could sweep masses of rock down into deep
gulfs, but could not sweep them up again, far less repeat this process
for hundreds of miles. Such blocks could only be transported by being
lifted up at the one place and set down at the other; and the only
agent we know of, capable of carrying such a freight, is the iceberg.
In this way, the bed of the sea in northern latitudes must be covered
with a thick stratum of mud and sand, plentifully interspersed with
boulders of all sizes, and its valleys must gradually be filled up as
year by year the deposit goes on.
But this is not all. The visible portion of an iceberg is only about
one-ninth part of the real bulk of the whole mass, so that if one be
seen 100 feet high, its lowest peak may perhaps be away down 800 feet
below the waves. Now it is easy to see that such a moving island will
often grate across the summit and along the sides of submarine hills;
and when the lower part of the berg is roughened over with earth and
stones, the surface of the rock over which it passes will be torn up
and dispersed, or smoothed and striated, while the boulders imbedded in
the ice will be striated in turn.
But some icebergs have been seen rising 300 feet over the sea; and
these, if their submarine portions sank to the maximum depth,
must have reached the enormous total height of 2700 feet--that is,
rather higher than the Cheviot Hills.[6] By such a mass, any rock or
mountain-top existing 2400 feet below the surface of the ocean would be
polished and grooved, and succeeding bergs depositing mud and boulders
upon it, this smoothed surface might be covered up and suffer no change
until the ocean-bed should be slowly upheaved to the light of day.
In this way, submarine rock surfaces at all depths, from the coast
line down to 2000 or 3000 feet, may be scratched and polished, and
eventually entombed in mud.
[Footnote 6: In the _American Journal of Science_ for 1843, p.
155, mention is made of an iceberg aground on the Great Bank of
Newfoundland. The average depth of the water was about 500 feet, and
the visible portion of the berg from 50 to 70 feet high, so that its
total height must have been little short of 600 feet, of which only a
_tenth_ part remained above water.]
[Illustration: Fig. 1. Iceberg grating along the sea-bottom and
depositing mud and boulders.]
And such has been the origin of the deep clay, which, with its included
and accompanying boulders, covers so large a part of our country. When
this arctic condition of things began, the land must have been slowly
sinking beneath the sea; and so, as years rolled past, higher and yet
higher zones of land were brought down to the sea-level, where floating
ice, coming from the north-west, stranded upon the rocks, and scored
them all over as it grated along. This period of submergence may have
continued until even the highest peak of the Grampians disappeared,
and, after suffering from the grinding action of ice-freighted rocks,
eventually lay buried in mud far down beneath a wide expanse of sea,
over which there voyaged whole argosies of bergs. When the process of
elevation began, the action of waves and currents would tend greatly to
modify the surface of the glacial deposit of mud and boulders, as the
ocean-bed slowly rose to the level of the coast line. In some places
the muddy envelope was removed, and the subjacent rock laid bare,
all polished and grooved. In other localities, currents brought in a
continual supply of sand, or washed off the boulder mud and sand, and
then re-deposited them in irregular beds; hence resulted those local
deposits of stratified sand and gravel so frequently to be seen resting
over the boulder clay. At length, by degrees, the land emerged from the
sea, yet glaciers still capped its hills and choked its valleys; but
eventually a warmer and more genial climate arose, plants and animals,
such as those at present amongst us, and some, such as the wolf, no
longer extant, were ere long introduced; and eventually, as lord of the
whole, man took his place upon the scene.[7]
[Footnote 7: The reader who wishes to enter more fully into the
geological effects of icebergs, should consult the suggestive section
on that subject in De la Beche's _Geological Observer_; also the
_Principles_ and _Visit to the United Stales_ of Sir Charles Lyell,
with the various authorities referred to by these writers.]
It is pleasant to mark, when once the true solution of a difficulty is
obtained, how all the discordant elements fall one by one into order,
and how every new fact elicited tends to corroborate the conclusion. In
some parts of the glacial beds, there occur regular deposits of shells
which must have lived and died in the places where we find them. From
ten to fifteen per cent, of them belong to species which are extinct,
that is to say, have not been detected living in any sea. Some of them
are still inhabitants of the waters around our coasts, but the large
majority occur in the northern seas. They are emphatically northern
shells, and get smaller in size and fewer in number as they proceed
southward, till they disappear altogether. In like manner, the palm,
on the other hand, is characteristically a tropical plant. It attains
its fullest development in intertropical countries, getting stunted
in its progress towards either pole, and ceasing to grow in the open
air beyond the thirty-eighth parallel of latitude in the southern
hemisphere, and the forty-fifth in the northern. So, too, the ivy,
which in our country hangs out its glossy festoons in every woodland,
and around the crumbling walls of abbey, and castle, and tower, is
nursed in the drawing-rooms of St. Petersburg as a delicate and
favourite exotic. In short, the laws which regulate the habitat of a
plant or an animal are about as constant as those which determine its
form. There are, indeed, exceptions to both. We may sometimes find a
stray vulture from the shores of the Mediterranean gorging itself on
sheep or lambs among the wolds of England,[8] just as we often see
"A double cherry seeming parted,
But yet an union in partition;"
or as we hear of a sheep with five legs, and a kid with two heads.
But these exceptions, from their comparative rarity, only make the
laws more evident. When, therefore, we find, in various parts of our
country, beds of shells in such a state of preservation as to lead
us to believe that the animals must have lived and died where their
remains are now to be seen, we justly infer that the districts where
they occur must at one period have been submerged. If the shells
belong to fresh-water species, it is plain that they occur on the site
of an old lake. If they are marine, we conclude that the localities
where they are found no matter how high above the sea must formerly
have stood greatly lower, so as to form the ocean bed. To proceed one
step further. If the shells are of a southern type, that is, if they
belong to species[9] which are known to exist only in wanner seas than
our own, we pronounce that at a former period the latitudes of Great
Britain must have enjoyed a more temperate and genial climate, so as
to allow southern shells to have a wider range northwards. If, on the
other hand, they are of an arctic or boreal type, we in the same way
infer that our latitudes were once marked by a severer temperature than
they now possess, so as to permit northern shells to range farther
southwards. This reasoning is strictly correct, and the truth involved
forms the basis of all inquiries into the former condition of the earth
and its inhabitants.
[Footnote 8: Two of these birds (_Neopron pecnopterus_) are stated to
have been seen near Kilve, in Somersetshire, in October 1825. One was
shot, the other escaped.]
[Footnote 9: There is not a little difficulty in reasoning
satisfactorily as to climatal conditions, from the distribution of
kindred forms. Even in a single genus there may be a wide range of
geographical distribution, so that mere generic identity is not always
a safe guide. Thus, the elephant now flourishes in tropical countries,
but in the glacial period a long-haired species was abundant in the
frozen north. I have above restricted myself entirely to _species_
whose habits and geographical distribution are already sufficiently
known.]
The evidence furnished by the northern shells in the boulder-clay
series is, accordingly, of the most unmistakable kind. These organisms
tell us that at the time they lived our country lay sunk beneath a sea,
such as that of Iceland and the North Cape, over which many an iceberg
must have journeyed, and thus they corroborate our conclusions, derived
independently from the deep clay and boulder beds and the striated
rock-surfaces, as to the glacial origin of the boulder-clay.
CHAPTER III.
How the boulder came to be one "Crag and tail"--Scenery of
central Scotland: Edinburgh--"Crag and tail" formerly
associated in its origin with the boulder-clay--This
explanation erroneous--Denudation an old process--Its
results--Illustration from, the Mid-Lothian coal-field--The
three Ross-shire hills--The Hebrides relics of an
ancient land--Scenery of the western coast--Effects of
the breakers--Denudation of the Secondary strata of the
Hebrides--Preservative influence of trap-rocks--Lost
species of the Hebrides--Illustration--Origin of the
general denudation of the country--Illustrative action
of streams--Denudation a very slow process--Many old
land-surfaces may have been effaced--Varied aspect of the
British Islands during a period of submergence--Illustration.
The scratched and grooved surface of the boulder was produced when
it was fast frozen in some iceberg, and driven gratingly across some
submarine summit, or stranded on some rocky coast-line. But, from its
rounded form, the stone had evidently undergone a long process of wear
and tear previous to its glacial journey. Probably it had hitherto lain
along a surf-beaten beach, where in the course of ages it had gradually
been worn into its present rounded shape. But how came it there? It
must originally have formed part of a flat sandstone bed, with many
other beds piled above it. By what agency, then, was this great pile
reduced to fragments?
The answer to these questions must be a somewhat lengthened one, for
the subject relates not to a few beds of rock hastily broken up and
dispersed, but to the physical changes of an entire country, carried on
during a vast succession of geological periods.
A phenomenon, known familiarly as "crag and tail," has long been
connected in its origin with the drift or boulder beds. Has my reader
ever travelled through central Scotland? If so, he must often have
noticed the abrupt isolated form of many of the hills, presenting a
mural front to the west, and a long sloping declivity to the east.
From the great number of isolated hard trap-rocks in this region, the
phenomenon is much better seen than in most other parts of the kingdom.
There is, for instance, the castle rock of Stirling, with its beetling
crag and castellated summit, which present so imposing a front to the
west. Many other examples are seen along the line of the Edinburgh and
Glasgow Railway. The range of hills south of Linlithgow, the singularly
abrupt basalt of Binny Craig, the long rounded ridge of Ratho,
the double-peaked crag of Dalmahoy, the broad undulation of woody
Corstorphine, are all examples more or less marked. Edinburgh itself
is an excellent illustration. The Calton Hill shows a steep front to
the town, while its eastern side slopes away down to the sea. Arthur's
Seat, in like manner, has a precipitous western face, and a gentle
declivity eastward. The Castle rock, too, shoots up perpendicularly
from the valley that girdles it on the north, west, and south, sinking
away to the east in a long slope--
"Whose ridgy back heaves to the sky,
Piled deep and massy, close and high."
East-Lothian presents several well-marked instances; in particular,
North Berwick Law and Traprain. A phenomenon so general must have had
some general origin, and it was accordingly attributed to the same
agency which produced the drift-clays and the striated rock-surfaces,
when these were believed to be the results of great diluvial action.
It would seem, however, that the phenomenon of crag and tail should
not be associated with the boulder-clay. The latter is undoubtedly a
newer Tertiary formation,[10] but the denudation[11] which produced
crag and tail must have been going on long ere the Tertiary ages had
begun. There is satisfactory evidence that large areas of our country
were planed down at a greatly more ancient period than that of even the
oldest of the Tertiary series. Thus, the whole area of the county of
Sussex suffered a very extensive denudation during the later Secondary
ages. The Hebrides had undergone a similar process previous to the
deposition of the Lias and Oolite, and the Greywacke hills of south
Scotland, previous to the formation of the Old Red Sandstone. There
seems thus to have been a general and continuous process of degradation
at work during a long succession of geological ages.
[Footnote 10: The reader is referred to the table of the geological
formations at the end of the volume for the relative position of the
beds described.]
[Footnote 11: _Denudation_ is a geological term used to denote the
removal of rock by the wasting action of water, whereby the underlying
mineral masses are _denuded_ or laid bare.]
The results of this long-continued action are of the most startling
kind. I have referred to the phenomenon of crag and tail as perhaps the
most readily observable. We must not fail to remember that the crag
which now stands up so prominently above the level of the surrounding
country, at one period lay buried beneath an accumulation of sandstone,
shale, or other strata, all of which have been carried away, so as
to leave the harder rock in bold relief, with a portion of the less
coherent strata sloping as a long tail from its eastern side. The crag,
too, is often breached in many places, worn down at one end, rounded
on the summit, and sometimes well-nigh ground away altogether, whilst
in front there is invariably a deep hollow scooped out by the current
when arrested by the abrupt cliff. In Fig. 2, _a_ represents a crag of
greenstone worn away and bared of the shales which once covered it;
_b_, the sloping "tail" of softer strata, protected from abrasion by
the resistance of the trap-rock, and covered by a deep layer of drift,
_d_; _c_ marks the hollow on the west side of the crag.
[Illustration: Fig. 2 "Crag and tail."]
But when we come to measure the actual amount of material that has been
carried away, we are lost in conjecture as to the vastness of the time
which such a process must have occupied. For instance, the coal-bearing
strata of Mid-Lothian must at one period have been connected with those
of Linlithgow and Stirling. At a subsequent date, the western area
subsided to form the Stirlingshire coal-basin, and the eastern area, in
like manner, sank down to form the coal-basin of Mid-Lothian, while the
intermediate portion stretched from east to west as a great arch, or,
as it is termed geologically, an _anticlinal axis_. Now, the whole of
this arch has been worn away, not a vestige of it remains, and yet its
upper or coal-bearing part was fully 3000 feet thick.[12]
[Footnote 12: This remarkable example of denudation was first described
by Mr. M'Laren, in his _Sketch of the Geology of Fife and the
Lothians_, a work in which the author showed himself to be in advance
of the science of his time.]
Let us take a small portion of this district, and endeavour to
calculate the amount of matter thus removed. The Pentland hills
form a chain stretching from near Edinburgh for some fourteen miles
southward, and having an average breadth of about two miles and a
half. They are formed chiefly of felspathic trap-rocks, resting upon
and interstratified with conglomerate apparently of Old Red age, which
in turn lies upon vertical Silurian slates. Before the Carboniferous
strata were thrown down by successive _faults_, they must have covered
these hills completely to a depth of not less than 6000 feet.[13]
From this small area, therefore, stratified sandstones, shales,
limestones, and coal, must have been removed to the enormous extent of
one billion, eight hundred and fifty-four thousand, four hundred and
sixty-four millions of cubic feet.
[Footnote 13: The actual depth of the Mid-Lothian coal-field, to the
base of the carboniferous limestone, is rather more than 3000 feet. It
is, perhaps, rather under than over the truth to allow 3000 feet for
the total thickness of beds from the limestone to the conglomerate of
Liberton, though, owing to the curved and contorted position of the
strata from Edinburgh to Stirlingshire, it is impossible to obtain a
measurement of their real thickness. I have attributed the isolation
of the Falkirk and Mid-Lothian coal-fields to the effect of faults
and general depressions of their areas. This was assuredly the case
in the latter coal-field, and probably in the former also. The trap
which occurs between them, though in great abundance, has certainly
not acted as an elevating agent. It occurs in beds among the strata,
and, judging from the number of associated tufas, appears to have been
to a considerable extent erupted while the lower carboniferous series
was forming. Mr. M'Laren, in his excellent work, p. 100, states his
opinion that the traps may have materially contributed to push up the
coal strata. A careful and extended examination of the district has
convinced me that this view is incorrect.]
But, perhaps, the most striking instances of denudation in the British
Islands are the three famous Ross-shire hills--Suil Veinn, Coul Mor,
and Coul Bheig. They are formed of piles of sandstone beds like tiers
of regular masonry, and reach a height of 3000 feet over the sea. The
sandstone of which they are composed must once have formed a bed or
set of beds fully 2000 feet thick, that covered the whole district for
many miles around. Yet of this extensive deposit there now exist only
a few isolated fragments. I have watched the sunshine and shadow of an
autumn sky resting alternately on these strange pyramidal hills, as
they towered in their giant proportions like the last remnants of a
mighty rampart that had stood the brunt of a long siege, and, breached
at last in many places, had been all but levelled to the ground. How
long-continued and how potent must that agency have been which could
cut down and disperse the massive barrier that flanked the western
coast of Ross-shire to a height of 2000 feet!
The Hebrides are but the shattered relics of an old land that had its
mountain-peaks and its glens, its streams and lakes, and may have
nursed in its solitude the red-deer and the eagle, but was never
trodden by the foot of man. A glance at the map is enough to convince
us of this. We there see islands, and peninsulas, and promontories,
and deep bays, and long-retiring inlets, as though the country had
been submerged and only its higher points remained above water. The
conviction is impressed more strongly upon us by a visit to these
shores. We sail through the windings of one of the "sounds," and can
scarcely believe that we are on the bosom of the salt sea. Hills rise
on all sides, and the water, smooth as a polished mirror, shows so
pure and limpid that in the sunshine we can see the white pebbles that
strew its bed many fathoms down. The eastern shore is often abruptly
interrupted by long-receding lochs edged round with lofty mountains,
and thus, where we had looked to see a deep heathy glen, with,
perchance, a white tree-shaded mansion in the far distance, and a few
dun smoking cottages in front, we are surprised to catch a glimpse of
the white sails of a yacht, or the darker canvas of the herring-boats.
We sail on, and soon a sudden turn brings us abruptly to the mouth of
the sound. A bold headland, studded around with rocky islets, rises
perpendicularly from the sea, bleak and bare, without a bush or tree,
or the faintest trace of the proximity of man. The broad swell of the
Atlantic comes rolling in among these rocks, and breaks in foam against
the grey cliffs overhead. In tempests, such a scene must be of the most
terrific kind. Wo to the hapless vessel that is sucked into the vortex
of these breakers, whose roar is sometimes heard at the distance of
miles! Even in the calmest weather the white surf comes surging in,
and a low sullen boom is ever reverberating along the shore. We see
the harder rocks protruding far into the sea, and often pierced with
long twilight caves, while the softer ones are worn into deep clefts,
or hollowed out into open bays strewed over with shingle. The sunken
rocks and islets, scarcely showing their tops above water, were all
evidently at one time connected, for, as we recede from the shore, we
can mark how the process of demolition goes on. There is first the
projecting ness or promontory, well-nigh severed from the mainland, but
still connected by a rude arch, through which the swell ever gurgles
to and fro. Then, a little farther from the shore, a huge isolated
crag, washed on all sides by the surge, raises its grey lichen-clothed
summit. A short way beyond, there is the well-worn islet whose surface
shelters neither lichen nor sea-weed, but is ever wet with the dash of
the waves. Further to the sea, the white gleam of the breakers marks
the site of the sunken rock. Thus, in the space of a hundred yards, we
may sometimes behold the progress of change from land to sea, and see
before us a specimen of that action which slowly but yet steadily has
narrowed and breached the outline of our western shores.[14]
[Footnote 14: I have endeavoured to illustrate the process of
denudation by a reference to breaker-action on the existing coast-line
of the Hebrides; but a strong current must have materially increased
the force of the ancient waves, and produced abrasion to some depth
below them.]
If we attempt to trace the connexions of strata among the Hebrides,
we shall be more fully impressed with the magnitude of the changes
which have been effected. Thus the Lias and Oolite occur in patches
along the shores of Mull, Morven, Ardnamurchan, Eigg, Skye, Raasay,
and Applecross. But though now only in patches, these formations must
once have extended over a considerable area, for they seem to form the
under-rock of the whole of the northern part of Skye, and are seen in
almost every lone island from Ardnamurchan Point to the Shiant Isles.
These scattered portions, often many miles distant from each other,
are the remnants of a great sheet of liassic and oolitic strata, now
almost entirely swept away, and are extant from having been covered
over with hard trap-rocks. But for these it may be doubted whether we
should ever have known that corals once gleamed white along the shores
of Skye, that the many-chambered ammonite swam over the site of the
Coolin Hills, that the huge reptilian monsters of these ancient times,
icthyosaurs and plesiosaurs, careered through the waters that laved
the grey hills of Sleat, and that forests of zamia and cycas, and many
other plants indicative of a warm climate, bloomed green and luxuriant
along the site of that strange mist-clad cliff-line, that shoots up
into the pinnacles of the Storr and Quiraing. It is curious to reflect,
that the records of these peaceful scenes have been preserved to us by
the devastating eruptions of volcanic forces; that the old lava-streams
which spread death through the waters along whose bed they travelled,
have yet been the means of protecting the districts which they wasted,
while those parts where they did not reach have been long since swept
away. It is allowable to believe, that in the portions of liassic
strata which have been destroyed there existed the remains of not a
few species, perhaps some genera, to be found nowhere else, and of
whose former existence there is now, by consequence, no trace. In the
small island of Pabba--a relic of the Scottish Lias--I found thirty-one
species, of which Dr. Wright has pronounced four to be new.[15] A
subsequent visit to the adjacent island of Raasay has increased the
list. In short, every patch of these Secondary rocks, if thoroughly
explored, might be found to yield its peculiar organisms. And in the
far larger area that has been carried away there existed, doubtless,
many more. We are accustomed to see individuals perish and their
remains crumble away, but the species still holds on. In the stratified
portion of the earth's crust, however, we mark how not merely
individuals have perished, but whole genera and species; but of these
the remains are still before us in the rocks; we can study their forms,
and, from a comparison with recent species and genera, can arrive at
some idea of their nature and functions. In this way, we are able to
picture the various conditions of the earth when these organisms lived
in succession upon its surface. Yet, we may readily conjecture, that
in ancient eras many tribes and genera of plants and animals lived
for ages, and then passed away without leaving any record of their
existence. Many circumstances might concur to prevent the preservation
of their remains. The species of the Hebrides were preserved in the
usual manner, but the cemetery in which their remains were entombed has
been washed away, and they can be seen nowhere else. It is as if on
some isolated country there had lived a race of men, tall Patagonians,
or swarthy Hottentots, or diminutive Laplanders, with a civilisation of
their own; owing to some change of climate the race gradually dwindled
down until it died out; eventually, too, the land settled down beneath
the sea with all its ruined cities and villages, which, as they reached
in succession the level of the waves, were torn up and dispersed, and
other races at last voyaged over the site of that old land, dreaming
not, that in bygone years fellow-mortals of an extinct type had
pastured their herds where now there rolled a widespread sea.
[Footnote 15: _Quart. Jour. Geol. Soc._, vol. xiv. p. 26.]
But to return. We have seen that the long-continued action of the sea
has been sufficient to breach and waste away the existing coast-line
of western Scotland. When, therefore, such results are produced by so
ordinary a cause, need we go to seek the agency of great debacles to
explain the denudation of other parts of the country? It is known that
at great depths currents have little effect upon the rocks which they
traverse, and that their action is greater as it nears the surface. To
account for the phenomena of crag and tail, and the general denudation
of the country, we may suppose the land to have been often submerged
and re-elevated. As hill after hill rose towards or sank below the
sea-level, it would be assailed by a strong current that flowed
from the west and north-west, until, in its slow upward or downward
progress, it got beyond the reach of the denuding agencies. In this way
the general contour of the land would be greatly though very gradually
changed. Hills of sandstone, or other material of feeble resistance,
would be swept away, the harder trap-rocks would stand up bared of the
strata which once covered them, deep hollows would be excavated in
front of all the more prominent eminences, and long declivities would
be left behind them.--(See Fig. 2.)
If my reader has ever visited the channel of a mountain-torrent--
"Imbres
Quern super notas aluere ripas"--
he must have noticed an exact counterpart to these appearances. When
the waters have subsided, the overflowed parts are seen to be covered
in many places with sand. Wherever a pebble occurs along the surface
of this sand, it has invariably a hollow before it on the side facing
the direction whence the stream is flowing, and a long tail of sand
pointing down the channel. If we watch the motion of the water along
its bed, the denuding agency may be seen actively at work. Every pebble
that protrudes above the shallow streamlet arrests the course of the
current, which is then diverted in three directions. One part turns
off to the right hand of the pebble, and cuts away the sand from its
flank; another part strikes off to the left, and removes the sand from
that side; while a middle part descends in front of the pebble, and,
by a kind of circular or gyratory movement, scoops out a hollow in the
sand in front. Behind the pebble the water is pretty still, so that the
sand remains undisturbed, and is further increased by the accumulation
above it of sediment swept round by the lateral currents. Now, in place
of the supposed stream, let us substitute the ocean with its westerly
current--for the pebble, a great trap-hill--for the sand, easily
friable shales and sandstones, and we have exactly the condition of
things which produced crag and tail.
This process of destruction must have been in progress during many
geological ages. We may suppose, that in that time the land often
changed level, sometimes rising far above the sea, and sometimes
sinking deep below it. We can well believe that the surface would
often be covered with vegetation; that plants, widely differing from
those which are now indigenous, clothed its hill-sides and shaded
its valleys; and that animals of long extinct forms roamed over its
plains or prowled amid its forests. When the country, in the lapse of
centuries, sank beneath the sea-level, all trace of these scenes would
eventually be effaced. The westerly currents would soon recommence the
process of degradation, uprooting the forests, devastating the plains,
wearing down the hills, and scooping out the valleys; and so, when the
ocean-bed, in the course of ages, became again dry land, it would arise
"another and yet the same." The little valley, where once, perchance,
the mastodon used to rest his massive bulk amid a rich growth of ferns,
shaded by the thick umbrage of coniferous trees, would emerge a deep
glen with bare and barren rocks on either side; the site of the hill
whereon herds of the gazelle-like anoplothere were wont to browse,
might reappear a level plain; the low-browed rock, under whose shadow
the ungraceful palæothere used of old to rest from the heat of the
noon-tide sun, might emerge a beetling crag shooting up several hundred
feet over the valley. It is by this repeated elevation and submergence,
carried on for many ages, that our country has acquired its present
configuration.
We can easily picture to ourselves the appearance which the British
Islands would thus at different periods present. At one time, nearly
the whole of England would be under water, with, however, a few islands
representing the higher peaks of Cornwall; others scattered over the
site of the West Riding of Yorkshire; and a hilly tract of land over
what is now Wales. Scotland must have existed in a sorely mutilated
state. A thick-set archipelago would represent the Cheviot Hills, and
the country south of the Forth and the Clyde; north of which there
would intervene a broad strait, with a comparatively large area of
undulating land beyond, stretching across what is now the area of
the Grampian Hills. A narrow fiord would run along the site of the
Caledonian Canal, cutting the country into two parts, and running
far into it on either side as deep lochs and bays. I have had such a
condition of things vividly recalled when on the summit of a lofty hill
in early morning, while the mists were still floating over the lower
grounds, and only the higher hill-tops, like so many islands, rose
above the sea of cloud. It was not a little interesting to cast the eye
athwart this changing scene, and mark how each well-known peak and
eminence looked when deprived of its broad sweep of base. What before
had always seemed an abrupt precipitous summit, now took the form of a
lonely rock or deep-sea stack, that might have served as a haunt for
the gull and the gannet. The long swelling hill rose above the mist as
a low undulating island, treeless and barren. It was easy to think of
that wide expanse of mist as the veritable domain of ocean, to picture
the time when these were veritable islands lashed by the surge, and
to conjure up visions of ice-floes drifting through the narrows, or
stranding on the rocks, amid a scene of wide-spread nakedness and
desolation.
CHAPTER IV.
Interior of the boulder Wide intervals of
Geology--Illustration--Long interval between the
formation of the boulder as part of a sand-bed,
and its striation by glacial action--Sketch of the
intervening ages--The boulder a Lower Carboniferous
rock--Cycles of the astronomer and the geologist
contrasted--Illustration--Plants shown by the boulder once
grew green on land--Traces of that ancient land--Its seas,
shores, forests, and lakes, all productive of material
aids to our comfort and power--Plants of the Carboniferous
era--Ferns--Tree-ferns--Calamites--Asterophyllites--
Lepidodendron--Lepidostrobus--Stigmaria--Scene in a ruined
palace--Sigillaria--Coniferæ, Cycadeæ--Antholites, the oldest
known flower--Grade of the Carboniferous flora--Its resemblance
to that of New Zealand.
I have likened the boulder to an old volume of the middle ages encased
in a modern binding. We have looked a little into the mechanism and
history of the boards; in other words, we have gone over the history of
the scratched surface of the boulder, of the clays and sands around it,
and of that still earlier cycle of denudation whereof the rock itself
is probably a relic. Before proceeding to open the volume itself, it
will be well that we clearly mark the wide interval in time between
the ages represented by the surface-striation and those indicated by
the interior of the boulder. When we proceed from the groovings on the
outside to the plants within, we pass, to be sure, over scarcely an
inch of space, but we make a leap over untold millenniums in point of
time. It is as if we had laid our hands on a volume of history which
had by some misfortune found its way into the nursery. The first page
that catches our eye relates the battle of the Reform Bill, and, on
turning the previous leaf, we find ourselves with Boadicea and her
woad-coloured soldiery. Now, if one utterly ignorant of the chronology
of the country were to be told that the volume related solely to one
people, he would at once see from the manners and customs delineated,
that the two pages referred to very different states of civilisation,
and consequently to widely-separated periods. But he could give no
account of how long an interval might have elapsed between the time
when London had its inhabitants massacred by Boadicea, and the time
when another generation of them was excited by the tardiness of King
William iv. He could form no conjecture as to what events might have
happened in the meanwhile. The interval might be a century or twenty
centuries, wherein the city might have been burnt down fifty times.
Clearly, if he wished to make himself acquainted with the intervening
history, he would have to betake himself to an unmutilated volume.
And just so is it with our boulder. We can easily believe, merely from
looking at it as it lies on its clayey bed, that a long time must have
elapsed between the time of its formation as part of a sandstone bed,
and the period of its transportation and striation by an iceberg. The
sand of which it is formed must have been washed down by currents, and
other sediment would settle down over it. It would take some time to
acquire its present hardness and solidity, while, in long subsequent
times, after being broken up and well-rounded by breaker or current
action, it may have lain on some old coast-line for centuries before it
was finally frozen into an ice-floe, and so freighted to a distance.
But the stone, with all its stories of the olden time, can tell us
nothing of this intervening period. It leads us from a dreary frozen
sea at once into a land of tropical luxuriance, and so, if we desire to
know anything of the missing portion of the chronology, we must seek it
elsewhere.
The Boulder-clay is one of the latest of geologic periods.[16] Beyond
it we get into Tertiary times, and learn from the caves of Yorkshire
how elephants, hyenas, rhinoceroses, hippopotami, bears, and wolves,
prowled over the rich valleys; while, from the quarries of the Isle of
Wight, we see how at an earlier time herds of uncouth palæotheres and
slimly-built anoplotheres browsed the plains of Old England. Beyond
the Tertiary ages come those of the Chalk, with its ocean that swarmed
with sea-urchins, terebratulæ, pectens, sponges, and many other forms.
Then arises the era of the Wealden, with its bosky land haunted by the
unwieldy iguanodon; the Oolite, with its land rich in a coniferous
flora, and tenanted by a race of small marsupial animals, and its seas
abounding in corals, encrinites of many a form, cidares, cuttle-fishes,
and ammonites. Further back still, come the times of the Lias, that
strange era in the history of our country, when reptiles huger than
those of the Nile swam the seas, and sped on wings through the air.
Then come the times of the Trias, when a vegetation still further
removed from existing types clothed the land, and frogs large as oxen
waddled along the shores. Then the times of the Permian, with its deep
sea tenanted by a meagre list of corals and shells, and by a type of
fishes that was slowly passing away. We arrive at last at the Coal or
Carboniferous period, to the older ages of which our boulder belongs.
[Footnote 16: For the names and succession of the rocks of which the
known part of the earth's crust is composed, see the Table at the end
of the volume.]
These eras may have been some longer, some shorter, but each had a
duration which, when tried by human standards, must be regarded as
immensely protracted. The cycles of astronomy are very vast, yet
I have often thought that the cycles of geology, though probably
of much less duration, impress us more forcibly with the antiquity
of our planet. The astronomer tells us of light that has taken two
millions of years to reach our earth, and of nebulæ that are millions
upon millions of miles distant, but these numbers are so vast that
we cannot bring ourselves to realize them. We _know_ that there is a
great difference between two millions and ten millions, but we cannot
fully _appreciate_ it, and so the periods of the astronomer, beyond a
certain point, cease adequately to impress us. So long as they can be
easily contrasted with our own standards of comparison, they have their
full force; but after that, every additional million, or ten millions,
or ten hundred millions, produces only a confused and bewildered sense
of immensity, and the comparative amount of each addition fails to be
realized. Will my reader forgive a homely illustration:--Some years
ago, I stood at the pier-head of one of our smaller sea-port towns, and
watched the sun as it sullenly sank behind the outline of the opposite
hills. The breadth of the channel, in the direction of sunset, was
several miles, but in the flush of evening one fancied he could almost
have thrown a stone across. The water lay unruffled by a ripple, and
reflected all the thousand varying tints that lighted up the sky. The
harbour, that had been a busy scene all evening, began to grow less
noisy, as one by one the herring-boats pushed out to sea. I found it
not a little interesting to mark, as the boats gained the open firth,
how the opposite coast-line gradually seemed to recede. The farther the
dark sails withdrew, the more remote did the adjacent shores appear,
until, as the last tinge of glory faded from the clouds, and a cold
grey tint settled down over the landscape, the hills lay deep in shade
and stretched away in the twilight as a dark and distant land from
whose valleys there rose troops of stars. The coast-line, as seen in
early evening, reminded me of the periods of the astronomer; as seen in
early night, it reminded me of the periods of the geologist. We fail to
appreciate the real duration of astronomical cycles, because they are
presented to us each as one vast period. They are not subdivided into
intervals, and contain no succession of events, by means of which, as
by milestones, we might estimate their extent; and so their unvaried
continuity tends to diminish the impression of their vastness, just
as the firth, without any islet or vessel on its surface, seemed
greatly narrower than it really was. For it is with time as it is with
space--the eye cannot abstractly estimate distance, nor can the mind
estimate duration. In either case, the process must be conducted by a
comparison with known standards. The geological periods exemplify the
same rule. They may not be greater, perhaps not so great, as those
revealed by astronomy, yet their vastness impresses us more, because we
can trace out their history, and see how step by step they progressed.
Thus, that the interval between the boulder-clay and the coal-measurer
was immense, we learn from the records of many successive ages that
intervened, in the same way that one began to perceive the real breadth
of the firth, by resting his eye on the succession of intervening
herring-boats. In the former case, the mind has ever and anon a sure
footing on which to pause in gauging bygone eternity; in the latter,
the eye had likewise a succession of points on which to rest in
measuring distance. Or, to return to a former illustration: Boadicea
lived eighteen hundred years ago, but who does not feel that the last
nine hundred years look a great deal longer than the first? The one
set has few marked incidents to fix the thoughts; the other is replete
with those of the most momentous kind. In the one, we have M meagre
list of conquerors and kings, from Julius Cæsar down to Athelstan; in
the other, events crowd upon us from the waning of the Saxon power down
through the rising glory of our country to the present plenitude of
its power and greatness. The early centuries, like the cycles of the
astronomer, pass through our mind rather as one continuous period; the
later centuries, like the cycles of the geologist, arrest our thoughts
by a succession of minor periods, and hence the idea of duration is
more vividly suggested by the diversified events of the one series,
than by the comparatively unbroken continuity of the other.
Let us now open the volume and try to decipher the strange legends
which it contains. On removing some of the upper layers of the boulder,
I found, as I have said, well-preserved remains of several kinds of
plants. One of them was ribbed longitudinally, with transverse notches
every three or four inches, us though a number of slender threads
had been stretched along a rod, and tied tightly to it at regular
intervals. Another, sorely mutilated, was pitted all over somewhat
after the fashion in which the confectioner punctures his biscuits. A
third had a more regular pattern, being prettily fretted with small
lozenge-shaped prominences that wound spirally round the stalk. Other
plants seemed to be present, but in a very bad state of preservation.
They were all jumbled together and converted into a black coaly
substance, in which no structure could be discerned.
These plants assuredly once grew green upon the land; but where now is
that land on which they flourished? Had it hills and valleys, rivers
and lakes, such as diversify our country? Was it tenanted by sentient
beings, and, if so, what were their forms? Did insects hum their way
through the air, and cattle browse on the plains, and fish gambol in
the rivers? Was the land shaded with forests, dark and rugged like
those of Norway, or fragrant as the orange-groves of Spain? What, in
fine, were its peculiar features, and how far did its scenery resemble
that of any country of the present day?
That old land has not entirely disappeared. Traces of it are found
pretty extensively in South Wales, in Staffordshire, around Newcastle,
and through central Scotland. Strange as it may seem, its forests
are still standing in many places. The fishes that disported in its
lakes, the insects that fluttered amid its woods, and the lizards that
crawled among its herbage, are still in part preserved to us. Nay,
more; we may sometimes see the sea-beaches of that ancient land pitted
with rain-drops, and roughened with ripple-marks, as freshly as if the
shower had fallen and the tide had flowed only yesterday. The peasants
along the Bay of Naples gathered grapes from the flanks of Vesuvius
for well-nigh seventeen centuries, before it was ascertained that they
daily walked over the site of buried cities, with temples, theatres,
and private houses still erect. It was many more centuries ere the
people of Great Britain discovered that not a few of their villages and
towns stood on the site of buried forests, and lakes, and seas. We have
now, however, become aware of the fact, and are making good use of it.
We dig into the earth and exhume these old forests to supply us with
light and fuel; we quarry into the ripple-marked shores which fringed
that old land, and build our houses with the hardened sand; we calcine
the ferruginous mud that gathered in its swampy hollows, and extract
therefrom our most faithful ally both in peace and war--metallic iron;
we burn the delicate corals and shells and lily-like zoophytes which
lived in the sea of that far-distant era, to enable us to smelt our
iron, to build our houses, and manure our fields; in short, every year
we are discovering some new and valuable material in the productions
of that period, or finding out some new use which can be made of the
substances already known. A more than ordinary interest, therefore,
attaches to the history of the land and sea which have furnished us
with so many aids to comfort as well as power; and we shall find, as we
go on, that that history is a very curious one.
I shall describe some of the more common plants and animals of the
period, that we may be able, in some measure, to look back through the
ages of the past, and see how these plants would appear when they cast
their broad shadow over river and lake, and how these animals would
have seemed to human eye in the twilight of the forest, in the sluggish
flow of the river, and in the stagnant waters of the lagoon.
The _Flora_, or vegetation of the Carboniferous era, differed
widely from any that now exists. With the exception of the highest
or exogenous class, it possessed representatives of all the existing
classes of the botanic scale, but in very strange proportions. The
number of species of carboniferous plants already found in Great
Britain amounts to about three hundred, amongst which the ferns are
especially abundant. Some of them seem to have been low-growing
plants, like the bracken of our hillsides, but others must have shot
up to the height of forest trees. We can recognise a few coniferous
and cycadaceous plants, a good many stems resembling the "horse-tail"
of our marshy grounds, and some of large size akin to the creeping
club-moss of our heaths; but there are still many to which there exist
no living analogues.
When we examine the roof of a coal-pit, or split open plates of shale
in a quarry of the coal-measures, we are struck with the similarity
which the ferns in the stone bear to those among our woods and hills.
One of the most common, and, at the same time, most elegant forms,
is the _Sphenopteris_ or wedge-leaved fern, of which a large list
of species is known. One of them (_S. crenata_) had a strong stem,
from which there sprung straight tapering branches richly dight with
leaflets. The leaflets--somewhat like minute oak-leaves--were ranged
like those of our modern ferns, along two sides of the stalk, in
alternate order, and tapered gently away to its outer extremity. The
effect of the whole is singularly rich, and one can well believe that a
garland of this ancient fern would have wreathed as gracefully around a
victor's brow as the parsley of Nemea or the laurel-leaves of Delphi.
Another plant of the same genus (_S. affinis_, Fig. 3) has leaflets
like the petals of the meadow-daisy, arranged in clusters along its
slim diverging stalks. From a collection and comparison of many
specimens, the late lamented Hugh Miller was enabled to make a drawing
of this fern as it must have appeared when it waved green along the
old carboniferous hill-sides. I enjoyed the privilege of going over
these specimens with him, and marked how, under a master-hand, piece
by piece fell into its proper place, and yielded up its evidence. His
restoration, which forms the frontispiece to his last work, is a very
beautiful one, and it is as true as it is beautiful.
[Illustration: Fig. 3. Sphenopteris affinis.]
[Illustration: Fig. 4. Pecopteris.]
[Illustration: Fig. 5. Cyclopteris.]
[Illustration: Fig. 6. Neuropteris.]
The _Pecopteris_ (Fig. 4, _P. heterophylla_) or comb-fern, is so called
from its stiff thick leaflets being in some species arranged along the
stalk like the teeth along the centre of a comb. Of all the plants
of the coal-measures this is the one that approaches most closely to
living nature. It appears to be almost identical with the _pteris_,
of which one species is well known as the bracken of our hill-sides.
Dr. Hooker figures together a frond of a New Zealand species (_P.
esculenta_) and a fossil frond from the Newcastle pits. They are
so similar as to be easily mistaken at first sight for drawings of
the same plant.[17] The _Neuropteris_ (as _N. gigantea_, Fig. 6) or
nerve-leaved fern, is remarkable for its strongly-defined venation. It
is scarcely, perhaps, so elegant in its outline as the _sphenopteris_,
or some of the other ferns. Its leaflets are large and thick, with an
oblong or rounded form, and arranged either singly along the frond
stem, or along secondary foot-stalks, which diverge from the main stem.
Of the latter kind, some of the species have a good deal of resemblance
to our _Osmunda regalis_ or royal fern. A species of the former class
(_N. cordata_) might readily enough be mistaken for the young leaves
of the _Scolopendrium_ or hart's-tongue, which hangs out its glossy
green amid the gloom of dank and dripping rocks. There are, besides,
several other genera of ferns in the Carboniferous strata, such as the
_Cyclopteris_ (_C. dilatata_, Fig. 5) or round-leaved fern, and the
_Odontopteris_ or tooth-fern. Most of these seem to have been lowly
plants, like the ferns of our own country. But there was another class
to which no analogue can be shown in Europe. They rose high over their
humbler congeners as lofty trees, and must be studied by a reference to
the existing tree-ferns of intertropical countries.
[Footnote 17: Hooker, _Mem. Geol. Surv._ vol. ii. part ii. p. 400.]
[Illustration: Fig. 7. Living Tree-fern.]
Tree-ferns flourish in warm climates, and are met with in Brazil, the
East and West Indies, New Zealand, &c. They rise sometimes to the
height of fifty or sixty feet, with a long tapering stem surmounted
by a dense crown of graceful fronds, and might easily be mistaken at
a little distance for palms. All the known species belong to the same
division (_Polypodiaceæ_) with the common polypodium of our road-sides.
In some genera, as the _alsophila_ of the East Indies, the trunk is
ribbed by long creeping branches, or rather rootlets, which descend to
the soil, giving the tree somewhat of the appearance so often seen in
old woods, where venerable fir-trees have been firmly encased by the
bearded stems of the ivy. Another genus, the _Cyathea_, has its stem
covered with oblong scars where leaves were attached, and a circle of
rich outspread fronds surmounts its summit. One of the coal-measure
tree-ferns seems to have resembled this recent type. It is named
the _Caulopteris_ or stalk-fern, and had a thick stem picturesquely
roughened by irregular oblong leaf-scars, that wound spirally from its
base to its point. No specimen has hitherto been found showing the
fronds in connexion with the stem, so that we are still ignorant of
the kind of foliage exhibited by this ancient tree. There can be no
doubt, however, that it was crowned with a large tuft of boughs that
cast their shadow over the sward below, and we may, perhaps, believe
that some of the numerous detached ferns found in the shales of the
coal-series, once formed part of this lofty coronal.
An important section of the carboniferous plants is embraced under
the generic name of _Calamites_. They had smooth jointed stems, like
reeds, and terminated beneath in an obtuse curved point (Fig. 8),
from which there sprang broad leaflets or rather rootlets. After many
years of research our knowledge of these plants is still very scanty.
Some of them have exhibited a highly-organized internal structure,
from which it appears that they consisted--first, of a soft central
cellular pith; second, of a thick layer of woody tissue; and third, an
external cylinder of strong bark, ribbed longitudinally, and furrowed
transversely. They have been ranked with the common horse-tail of our
ponds, but they would rather appear to belong to a higher family. The
breadth of the stem is very various, some specimens being a foot or
more in diameter, others scarcely half an inch. From the discoveries of
Professor Williamson and Mr. Binny of Manchester, it seems not unlikely
that what we call calamites may be really the inner core of a plant not
yet named, just as a set of fossils were long called _sternbergiæ_,
before they were discovered to be really the pith of coniferous trees.
With regard to the branches of the calamites, Brongniart's conjecture
may be true, that they exist among the group of plants called
_asterophyllites_. It is not unlikely that many dissimilar plants have
been grouped together as calamites, and, on the other hand, that plants
allied to the typical species have been thrown into separate genera.
For it requires but a slight acquaintance with the vegetable kingdom to
know how many forms analogous parts of the same plant may assume, and
how impossible it would often be to guess the real relationship of such
varieties if they were not found growing together on one plant.
[Illustration: Fig. 8. Terminal portion of a calamite stem.]
[Illustration: Fig. 9.[18]]
[Footnote 18: The fossil given in Fig. 9 is named by Lindley (_Foss.
Flo._ t. 15, 16), _Calamites nodosus_. He admits, however, that it was
not found in actual contact with a calamite stem. It has exactly the
contour of an asterophyllite, and might, perhaps, be referred to that
genus. It is inserted here that the reader may see the general form of
the asterophyllites, and the close relationship that subsists between
these plants and the calamites.]
A remarkably graceful class of the coal-plants are known as
_asterophyllites_. They had slim fluted and jointed stalks, apparently
of humble growth. From each of the joints there sprang two thin
opposite branches with stellate clusters of leaflets arranged round
them at equal distances. If the reader will take a young rush-stalk,
and string along it a number of the flowers of the little star-wort,
keeping them a little distance apart, he may form some idea of the
appearance of a single branch of the star-bearing _asterophyllite_.
Some of the plants embraced under this genus are conjectured to have
been aquatic, spreading out their clusters of leaflets in the green
sluggish water of stagnant pools; but many of them are evidently
related to the calamites, and may possibly have formed part of these
plants.
Whoever has rambled much in a coal-country, scrambling through briars
and brambles in old quarries, or threading his way among the rocks
of river-courses, must often have noticed, on the exposed surface of
sandstone blocks, dark ribbon-like bands fretted over with little
diamond-shaped knobs. They are so common in some districts, that you
can scarcely light upon a piece of sandstone which does not show one
or more. They belong to a carboniferous plant known as _lepidodendron_
(Fig. 10) or scaly tree, from the peculiar style of ornamentation which
adorned its bark. Its structure and affinities have puzzled botanists
not a little. A well-preserved specimen reminds one of the appearance
presented by a twig of the Scotch fir, when stripped of its green
spiky leaflets. The scars thus left at the base of the leaflets are
of a wedge-like form, and run spirally up the branch in a manner very
like those on the branches of lepidodendron; and it was accordingly
supposed at one time that the latter plant belonged, or at least was
allied, to the conifers. But the branches of lepidodendron possessed a
peculiarity that is shared in by none of our present coniferous trees.
They were what botanists call _dichotomous_,--that is, they subdivided
into two equal branches, these again into other two, and so on. Their
internal texture,[19] too, differed from that of any known conifer.
The only tribe of existing plants with which the lepidodendron seems
to bear comparison, are the _Lycopodiaceæ_, or club-mosses, of which
we have several species in the moor-lands of our own country. They
are low trailing plants, with moss-like scaly branches, bearing at
their ends shaggy little tufts, whence the popular name of the genus.
In warmer climates, they are both more numerous and attain a larger
size, sometimes standing erect to about the height of an ordinary
gooseberry-bush. But though the lepidodendron appears to have been
allied to these plants in structure, it greatly differed from them in
dimensions. The club-mosses of the coal-measures shot up as goodly
trees, measuring fifty feet and upwards in height, and sometimes
nearly five in diameter. Their general effect must have been eminently
picturesque. A shaggy covering of green spiky leaflets bristled over
their multitudinous pendant boughs; and where on the older stems these
leaflets had decayed and dropped off, the outer bark was laid bare,
fretted over with rows of diamond-shaped or oval scars, separated
by waving lines of ridge or furrow, that wound spirally round the
stem. From not a few of the branches there sprang oblong hirsute
cones called _lepidostrobi_ (Fig. 11), which bore the sporangia, or
seed-cases. These cones are of frequent occurrence in the shales of
the coal-measures, and may be readily recognised. They had a central
axis round which the oblong sporangia were built, the whole being
protected externally by a thick covering of pointed scales, imbricated
like the cone of the Scotch fir. The leaflets of lepidodendron, called
_lepidophylla_, were broader than those of the Scotch fir, and had a
stout mid-rib, which must have given them a rigidity like that of the
araucarian pine a plant they may also have resembled in the dark glossy
green of its leaves.
[Footnote 19: See Hooker, _Mem. Geol. Surv._ vol. ii. part ii. p. 436.]
[Illustration: Fig 10. Lepidodendron Sternbergii.]
[Illustration: Fig. 11. Lepidostrobus.]
Of all the common coal-measure plants, there is perhaps none so
abundant as that known by the name of _stigmaria_, or punctured-stem.
It is found spreading out its rootlets for several yards in beds of
shale and under-clay, and sometimes even limestone,[20] while, in
many sandstones, fragments of its blackened stems lie as thickly
strewn as twigs among the woods in autumn. I have said that several
of the plants above described have greatly puzzled botanists. None
of them, perhaps, has given rise to so much conjecture and variety
of opinion as the stigmaria. The history of the discussion regarding
its nature and affinities, would be not a little interesting as an
illustration of the slow hindered progress often attendant on the
researches of science, and an instance of how a few simple facts are
sometimes enough to overturn the most plausible theories and probable
conjectures. Many thousands of specimens had been examined ere one was
found that revealed the true nature of the stigmaria. It was by some
imagined to be a soft succulent marshy plant, consisting of a number
of long branches radiating from a sort of soft disk, like spokes from
the centre of a wheel. Analogies were suggested with dicotyledonous
tribes, as the _cacti_ and _euphorbiæ_, though it was at the same time
admitted that the ancient plant presented appearances which seemed very
anomalous.
[Footnote 20: The fresh-water limestone of Mid-Calder abounds in
long trailing stems and rootlets of stigmaria, mingled with other
terrestrial plants, and shells of _cyprides_.]
[Illustration: Fig. 12 Stigmaria rootlets springing from Sigillaria
stem.]
In the course of an extensive survey of the coal-field of South
Wales, Mr. (now Sir William) Logan ascertained the important fact,
that each coal-seam is underlaid by a bed of clay, in which the stems
of stigmaria, branching freely in all directions, may be traced to
the distance of many feet or even yards. They were recognised as
undoubtedly occupying the site on which they grew, and consequently
each coal-seam was held to rest upon an ancient soil. Some years
afterwards, in making a cutting for the Lancaster and Bolton Railway,
several upright massive stems belonging to a plant called _sigillaria_,
were found to pass downwards into true stigmaria stems (Fig. 12).
There could be no doubt that they were different parts of one and the
same plant. This fact has since been abundantly demonstrated from the
Nova Scotia coal-field. Many sigillariæ have been found there passing
down into the fire-clay below, where they branch out horizontally as
true stigmatiæ. It is evident, therefore, that the stigmaria was the
under-ground portion of a plant, which, judging from the nature of the
soil, and the free mode in which the tender rootlets branched off,
appears to have lived in aquatic or marshy stations.
[Illustration: Fig. 13. Stigmaria.]
The stigmaria is too well marked to be readily confounded with any
other coal-measure plant. It had a rounded stem, seldom more than
four or five inches across, which was marked by a series of circular
tubercules with a puncture in the centre, arranged in spiral lines
round the stem. Each of these tubercules is surrounded, in ordinary
specimens, by a circular depression,[21] and the whole plant (if
one may use the comparison) looks as if it had been smitten with
small-pox. From the hollow in the centre of each protuberance, there
shot out a long round rootlet, formerly thought to be a leaf, and
since the tubercules are pretty thickly set, the stigmaria must have
had a somewhat hirsute appearance as it crept through the mud. It
would resemble a thick bearded stem of ivy, save that the fibres,
instead of running up two sides, were clustered all round it. Along
the centre of the root, there ran a woody pith of a harder and more
enduring texture than the surrounding part of the plant. The space
between the outer tuberculed rind and the inner pith, seems to have
been of a soft cellular nature, and to have decayed first, for the pith
is sometimes hollow, and may not unfrequently be seen at a distance
from the centre, and almost at the outer bark--a circumstance that
seems only explicable on the supposition, that while the surrounding
portions were decaying, the firmer pith altered its position in the
hollow stem, sinking to the lower side, if the plant lay prostrate, and
that it did not itself begin to decay until the interior of the stem
had been at least partially filled up with sand or mud, or fossilized
by the infiltration of lime. From the root of the sigillaria, which
has a curious cross-shaped mark on its base, the stems of stigmaria
strike out horizontally, first as four great roots which subdivide as
they proceed. Their subdivisions are dichotomous, each root splitting
equally into two, and thus they want that intricate interlacing of
rootlet which is so familiar to us. The whole disposition of these
under-ground stems is singularly straight and regular, leading us to
believe that they shot out freely through a soft muddy soil.
[Footnote 21: Such is the usual aspect of the plant. But as the stems
have been, for the most part, greatly flattened by the pressure of
the superincumbent rocks, the sharpness of the pattern has been much
effaced. In some specimens described by Dr. Hooker, as having been
found in an upright position, the external ornamentation presents an
appearance somewhat different. What in the common specimens stand out
as tubercules, are there seen to be deep circular cavities, in which
the shrunk flagon-shaped bases of the rootlets are still observable.
(See above, Fig. 13 _b_, which is taken from one of Dr. Hooker's
plates. For a detailed description of the structure of stigmaria, see
the paper above referred to in the _Geological Survey Memoirs_.) A
very ornate species is mentioned by the late Hugh Miller, in which
each tubercule formed the centre of a sculptured star, and the whole
stem seemed covered over with flowers of the composite order. And what
is, perhaps, still more curious, the stem was seen to end off 7 in an
obtuse point, tuberculed like the rest of the plant.--_Testimony of the
Rocks_, p. 461.]
Some time ago I chanced to visit the remains of what had once been
a royal residence, and still looked majestic even in decay. It gave
a saddened pleasure to thread its winding stairs, and pass dreamily
from chamber to hall, and chapel to closet; to stand in its gloomy
kitchens, with their huge fire-places, whose blackened sides told of
many a roaring fagot that had ruddied merry faces in days long gone
by; to creep stealthily into the sombre dungeons, so dank, earthy, and
cold, and then winding cautiously back, to emerge into the light of the
summer sun. The silent quadrangle had its encircling walls pierced with
many a window, some of which had once been richly carved; but their
mullions were now sorely wasted, while others, with broken lintels and
shattered walls above, seemed only waiting for another storm to hurl
them among the roofless chambers below. In the centre of the court-yard
stood a ruined fountain. It had been grotesquely ornamented with heads
of lions and griffins, and was said to have once run red with wine. But
it was silent enough now; the hand of time, and a still surer enemy,
the hand of man, had done their worst upon it; its groined arches
and foliaged buttresses were broken and gone, and now its shattered
beauty stood in meet harmony with the desolation that reigned around.
I employed myself for a while in looking over the fragments, marking
now the head of some fierce hippogryph, anon the limbs of some mimic
knight clad in armour of proof, and ere long I stumbled on a delicately
sculptured _fleur-de-lis_, that might have surmounted the toilet-window
of some fair one of old. Turning it over, I found its unhewn side
exhibited a still more delicately sculptured stigmaria. The incident
was certainly simple enough, perhaps even trifling. And yet, occurring
in a spot that seemed consecrated to reverie, it awoke a train of
pleasant reflection. How wide the interval of time which was bridged
across in that sculptured stone! Its one side carried the mind back
but a few generations, the other hurried the fancy away over ages and
cycles far into the dim shadows of a past eternity. The one told of a
land of flowers, musical with the hum of the bee and the chantings of
birds, and gladdened by the presence of man; the other told of a land
luxuriant, indeed, in strange forms of vegetation--huge club-mosses,
tall calamites, and waving ferns--yet buried in a silence that was only
broken fitfully by the breeze as it shook the spiky catkins or the
giant fronds of the forest. The _fleur-de-lis_ recalled memories of
France--the sunny land of France--which stood out so brightly in the
dreams of our school-days; the stigmaria conjured up visions of a land
that was never gazed on by human eye, but rolled its rich champaign
during the long ages of the Carboniferous era, and sometimes rises up
dimly in the dreams of our maturer years. Between these two epochs
how many centuries, how many cycles must have slowly rolled away! The
_fleur-de-lis_ was carved but yesterday; the stigmaria flourished when
the earth was young, and had seen scarcely a third part of its known
history.
I have said that the stem of the stigmaria is called sigillaria.
The name may be translated _signet-stem_,[22] and has reference to
one of the distinguishing peculiarities of the plant. About twenty
British species are enumerated, some of them very dissimilar, yet
they all agree in having long fluted stems with parallel rows of
prominent seal-like tubercules. The sigillaria differed so widely in
its whole contour and ornamentation from every living plant, that it
is impossible to convey an idea of its form by reference to existing
vegetation. Some of the species, as _S. organum_ (Fig. 14), had their
trunks traversed longitudinally by broad ridges separated by narrow
furrows. Along the summit of each ridge there ran a line of tubercules,
set regularly at distances varying from a third or a quarter of an inch
to close contact. One may sometimes see no unfair representation of
the bark of this ancient tree, when looking at a newly ploughed field
in spring-time, having each of its broad ridges dotted with a row of
potato sacks. Other species, while exhibiting the same plan, differed
not a little in the details. In some the tubercules are round, in
others angular, and in a third set double or kidney-shaped. In some
they are far apart, in others they are strung together like a chain of
beads. Sometimes they exist as mere specks, while occasionally they
broaden out so as to equal in width the ridge that supports them.
One species (_S. reniformis_), instead of the broad ridge and narrow
furrow, exhibits an arrangement exactly the reverse. It looks not
unlike a cast of the species first described, save that its broad flat
furrows support rows of much larger tubercules. The breast of a lady's
chemisette, with a thick-set row of buttons down each plait, would
be somewhat like this species of sigillaria, with this difference,
however, that the buttons on the plant were of a form that does not
appear as yet to have come into fashion among the fair sex. Yet they
had no little elegance, and like many other objects in the geological
storehouse, might be a useful model for our students of design. They
were neither round nor quite oval, but rather of a kidney-shape, or
like a double cherry.
[Footnote 22: The word sigillaria is really plural, and was used by the
Romans to denote the little images which friends were wont to present
to each other at the end of the Saturnalia. They answered pretty nearly
to christmas-boxes and new year's gifts among ourselves. It is not
uninteresting thus to find among the hard dry names of science, one
that two thousand years ago was synonymous with all the kindliness of
friendship.]
[Illustration: Fig. 14. Sigillaria, with black carbonized bark
partially removed.]
There can be no doubt that these tubercules must once have supported
leaflets. They are true leaf-scars, like those on the Scotch fir, and
the lozenge-shaped knobs on the bark of lepidodendron. But of the form
of these leaves we are still in ignorance, for no part of the plant,
save the stem and roots, has yet been found. The sigillaria must have
been a tree that could not long withstand maceration, for not only are
its leaves gone, but, in many cases, the outer bark has partially or
wholly decayed, leaving a scarcely distinguishable mass of carbonized
matter.[23] When this outer rind is peeled off, the inner surface of
the stem is seen to be ridged, furrowed, and tuberculed in the same
way, but the markings are much less distinct than on the outside. The
bark sometimes attains the thickness of an inch, and is always found
as a layer of pure coal enveloping the stem where it stands erect, or
lying as a flat cake without any central cylinder where the stem is
prostrate. (See Fig. 14.)
[Footnote 23: Another proof of the looseness of the texture of this
ancient vegetable may be gathered from the almost invariable truncation
of even the largest erect stems; they are snapped across at the height
of a few feet from their base. The famous "Torbanehill Mineral"
contains many such fragmentary stems, often of considerable thickness.
Their interior consists of the same material as the surrounding bed,
and displays many dissevered plants that may have been washed into
the decaying trunks. For the internal structure of sigillaria see Dr
Hooker's _Memoir_, and the authorities therein cited.]
Another remarkable feature in this carboniferous plant is that it
appears to have had no branches along its stem. Trunks have been found
four and five feet in diameter, and have been traced to a distance of
fifty, sixty, and even seventy feet, without any marks of branches
being detected. Brongniart examined the portion of one stem, which,
at its thicker end, had been broken across, but still measured a
foot in breadth. It ran for forty feet along the gallery of a mine,
narrowing to a width of not more than six inches, when it divided into
two, each branch measuring about four inches across. The sigillaria
stems, accordingly, must have shot up, slim and straight, to a height
of sometimes seventy feet before they threw out a single branch. We
know nothing of the coronal of these strangely-formed trees. From
Brongniart's observations, it would seem that the upper part of the
stem, like that of the lepidodendron, was dichotomous, that is, it
branched out into two minor stems; but how these were disposed is
unknown. We are wholly ignorant, too, of the foliage of these branches,
though, from the general structure of the plant, as well as from the
number of fern-fronds often found around the base of the stems, it has
been conjectured that the sigillaria was cryptogamous, and, like the
tree-ferns, supported a group of sweeping fronds. If so, it differed in
many respects from every known member of the cryptogamic tribes.
Putting together, then, all that we know of the exterior of the
sigillaria, we find that it was a tall slender tree, with, palm-like,
a clump of foliaged branches above, its stem bristling thickly, in at
least its upper part, with spiky leaves, and its roots equally hirsute,
shooting out to a distance of sometimes forty feet through the soft
muddy soil. Future researches may bring us better acquainted with this
ancient organism. In the meanwhile, enough of it is known to mark it
out as one of the most ornate forms of vegetation that the world has
ever seen.
In addition to the above, the coal strata have yielded many other
fragmentary remains, to which names have been given, but of which very
little is known. It is pleasant, amid such a wide sea of doubt and
uncertainty, to alight upon some well-known form of whose affinities
there can be no question, since it still finds its representatives in
living nature. Of such a kind are the coniferous stems occasionally met
with in the sandstones of the coal-measures. .
It is now many years since the operations of the quarryman in the
carboniferous sandstones of Edinburgh and Newcastle disclosed the
remains of huge gnarled trunks deeply imbedded in the rock. The
neighbourhood of the latter town yielded, in 1829,[24] the stem of
a tree seventy-two feet long, without branches, but roughened with
numerous knobs, indicative of the places whence branches had sprung.
At Craigleith, near Edinburgh, a trunk thirty-six feet long, and three
feet in diameter at the base, was disinterred in the year 1826. Since
then, several others have been found in the same neighbourhood; some
of them sixty and even seventy feet in length, and from two to six in
breadth. They were, for the most part, stripped of roots and branches,
and lay at a greater or less angle among the white sandstone beds,
which they cut across obliquely. It was unknown for some time to what
division of the vegetable kingdom these trunks should be referred.
Their irregular branched surface and undoubted bark indicated a higher
kind of structure than that possessed by any of the other carboniferous
plants; but the conjecture remained unverified until an ingenious
and beautiful method was discovered of investigating their internal
organization. Two Edinburgh geologists, Mr. Nichol and Mr. Witham,
succeeded in obtaining slices of the plants sufficiently transparent to
be viewed under the microscope by transmitted light, and in this way
their true structure was readily perceived. The method of preparing
these objects was simply as follows:--A thin slice of the plant to be
studied was cut by the lapidary, or detached by the hammer. One side
having been ground down smooth, and polished, was cemented by Canada
balsam to a piece of plate-glass, and the upper surface was then ground
down and polished in like manner, so as to leave the slice no thicker
than cartridge-paper.[25] When the preparation was then placed under
a magnifying power, the minute cells and woody fibre of the plant
could be detected as clearly as those of a recent tree. The Craigleith
fossils were in this way recognised as belonging to the great
coniferous family, and to that ancient[26] division of it which is, at
the present day, represented by the pine of Norfolk Island--"a noble
araucarian, which rears its proud head from 160 to 200 feet over the
soil, and exhibits a green and luxuriant breadth of foliage rare among
the coniferæ."[27] Some of these plants have yielded faint traces of
the annual rings shown so markedly in the cross section of our common
forest-trees; whence it would appear, that even as far back as the
times of the coal-measures, there were seasons of alternate heat and
cold, though probably less defined than now.
[Footnote 24: Witham's _Foss. Veget._ p. 31.]
[Footnote 25: For a more detailed description of the process, see
Witham's _Foss. Veget._ p. 45.]
[Footnote 26: The solitary lignite of the Lower Old Red Sandstone,
seems to have been araucarian. Miller's _Footprints of the Creator_, p.
203.]
[Footnote 27: _Footprints of the Creator_, p. 192.]
These coniferous trees do not appear to occur among the erect stems of
the coal-beds, at least they are very rare in such a position. Their
more usual appearance is that of drifted, branchless trunks, imbedded
along with other fragmentary plants in deep strata of sandstone.
They probably grew on higher ground than the swamps which supported
the sigillariæ and their allies, and might have been carried down by
streams, freighted out to sea, and so deposited among the sediment that
was gathering at the bottom.
The remains of cycadaceous plants have been described among the
vegetation of the coal-measures; but only fragments have as yet been
found. The modern _Cycadeæ_ are low shrubs or trees, with thick stems
of nearly uniform breadth, crowned with a dense clump of spreading
fronds which resemble both those of the palms and the ferns. They are
natives of the warmer regions of both hemispheres.
So long ago as the year 1835, Dr. Lindley figured a flower-like
plant, to which he gave the name of _Antholites_, ranking it among
the _Bromeliaceæ_, or pine-apple group. It was afterwards suspected
by Dr. Hooker to belong rather to the coniferæ; and he supposed that
the so-called flowerets might be really tufts of young unexpanded
leaves. An examination of a more perfect specimen, however, has induced
that distinguished botanist to alter his convictions and return to
the original decision of Lindley, that the antholites are really
flowers.[28] In Fig. 15, therefore, which represents one of these
coal-measure fossils, the reader beholds the oldest flower that has
yet been found; and surely it is of no little interest to know, that
amid the rank, steaming forests of the Carboniferous era, with all
their darkness and gloom, there were at least some flowers--flowers,
too, that were allied to still living forms, and breathed out a rich
aromatic fragrance.
[Footnote 28: See Dr. Hooker's remarks in the Supplement to the fifth
edition of Lyell's Manual, p. 31.]
[Illustration: Fig. 15.--Antholites.]
In fine, from all the genera and species of plants that have been
detected in the strata of the coal-measures, it would appear that the
flora of that ancient period was in a high degree _acrogenous_--that
is to say, consisted in great measure of ferns, club-mosses, and other
members of the great group of plants known as _acrogens_. This word
literally means _top-growers_, and is applied to those plants which
increase in height, but not in width, since they attain at first
nearly their ultimate diameter. Such plants occupy a low position in
the botanical scale. Mingled with the numerous genera of carboniferous
ferns and club-mosses, we find the remains of a much higher grade
of vegetation--that of the _gymnogens_, or plants that bear naked
seeds--such as the firs and pines. There also seem to have been a
few _endogenous_ flowering plants. Viewing, then, this flora on the
whole, it presents us with many striking resemblances to certain
botanical regions of the present day. Many of the tropical islands
abound in ferns, and contain very few flowering plants. But New Zealand
affords perhaps the closest parallel. That island is in certain parts
highly mountainous, its loftiest summits being covered with glaciers.
The hills throughout large districts are bare, or covered with a
scanty herbage, while in other localities they are densely clothed
with forests of pine, beech, and other trees. These forests sweep
on to the lower grounds, where they are replaced by a thick growth
of fern and flax-plant intermingled with dragon-trees and graceful
tree-ferns, while the more swampy regions support a rich profusion of
reeds and rushes. Such a condition of things affords a close parallel
to the probable vegetation of the Carboniferous period--an immense
preponderance of ferns and arborescent acrogens, with an intermixture
of large coniferous trees. From the general scantiness of a flora
where ferns predominate, it has been argued that the swamps of the
coal-measures nourished a luxuriant repetition of comparatively few
species; and this hypothesis also receives confirmation from the
vegetation of New Zealand. Another deduction founded on the resemblance
of the ancient to the modern flora, refers to the conditions of heat
and moisture. It has been inferred that the climate of the coal period
was equable and humid, like that of New Zealand--a supposition much
more natural and simple than that, once so much in vogue, of a heated
atmosphere densely charged with carbonic acid gas. That the air of the
Carboniferous period differed in no material respect from the air of
the present day, seems at last proved by the remains of air-breathing
animals having been found among the coal-beds; and there seems no
reason why the higher mountain-tops of the same epoch may not have been
clothed with glaciers as those of New Zealand are. As yet we have no
evidence of the fact, but it is by no means beyond the possibility of
proof.[29]
[Footnote 29: See Professor Ramsay's suggestive Memoir on Permian
Breccias in _Quarterly Journal of the Geological Society_, vol. xi. p.
185.]
CHAPTER V.
Scenery of the carboniferous forests--Contrast in the
appearance of coal districts at the present day--Abundance
of animal life in the Carboniferous era--Advantages
of palæontology over fossil-botany--Carboniferous
fauna--Actiniæ--Cup-corals--Architecture of the present
day might be improved by study of the architecture of the
Carboniferous period--Mode of propagation of corals--A
forenoon on the beach--Various stages in the decomposition of
shells--Sea-mat--Bryozoa--Fenestella--Retepora--Stone-lilies--
Popular superstitions--Structure of the stone-lilies--Aspect
of the sea-bottom on which the stone-lilies
flourished--Sea-urchins--Crustacea, their high
antiquity--Cyprides--Architecture of the Crustacea and
mollusca contrasted--King-crabs.
The forms of vegetation that flourished during the Carboniferous era
seem to have been in large measure marshy plants, luxuriating on low
muddy delta-lands, like the cypress-swamps of the Mississippi, or the
Sunderbunds of the Ganges. We can picture but faintly the general
scenery of these old forests from the broken and carbonized remains
that have come down to us. But though perhaps somewhat monotonous
on the whole, it must have been eminently beautiful in detail. The
sigillariæ raised their sculptured stems and lofty waving wreaths of
fronds high over the more swampy grounds, while a thick underwood of
ferns and star-leaved asterophyllites clustered amid the shade below.
The lepidodendra shot forth their spiky branches from the margin of
green islets, and dropped their catkins into the sluggish water that
stole on among the dimpled shadows underneath. Tree-ferns spread out
their broad pendant fronds, and wrapt the ground below in an almost
twilight gloom, darker and deeper far than that
"Hospitable roof
Of branching elms star-proof,"
which rose so often in the visions of Milton; or that "graceful arch"
so exquisitely sung by Cowper, beneath which
"The chequered earth seems restless as a flood
Brushed by the wind. So sportive is the light
Shot; through the boughs, it dances as they dance,
Shadow and sunshine intermingling quick,
And darkening and enlightening, as the leaves
Play wanton, every moment, every spot."
Thickets of tall reeds rose out of the water, with stems massive as
those of our forest-trees, encircled at regular distances by wreaths of
pointed leaflets, and bearing on their summits club-like catkins. Far
away, the distant hills lay shaggy with pine-woods, and nursed in their
solitudes the springs and rivulets that worked a devious course through
forest, and glen, and valley, until, united into one broad river, they
crept through the rich foliage of the delta and finally passed away out
to sea, bearing with them a varied burden of drift-wood, pine-trees
from the hills, and stray leaves and cones from the lower grounds.
How different such a scene from that now presented by the very same
areas of country! These old delta lands are now our coal-fields, and
have exchanged the deep stillness of primeval nature for the din and
turmoil of modern mining districts. In these ancient times, not only
was man uncreated, but the earth as yet lacked all the higher types of
vertebrated being. None of the animals that we see around us existed
then; there were no sheep, nor oxen, horses, deer, nor dogs. Neither
were the quadrupeds of other lands represented; the forests nourished
no lions or tigers, no wolves or bears, no opossums or kangaroos. In
truth, the land must have been a very silent one, for we know as yet of
no animated existence that could break the stillness, save perchance
some chirping grasshopper, or droning beetle, or quivering dragon-fly.
No bee hummed along on errands of industry; it is doubtful, indeed,
whether honey-yielding flowers formed part of the carboniferous flora;
no lark carolled blithely in the sky, nor rook croaked among the
woods. All was still; and one might, perhaps, have stood on some of
those tree-crested islets, and heard no sound but the rippling of the
water along the reedy and sedgy banks, and the rustling of the gloomy
branches overhead.
To one who muses on these bygone ages it is no unimpressive situation
to stand in the midst of a large coal district and mark its smoking
chimneys, clanking engines, and screaming locomotives, its squalid
villages and still more squalid inhabitants, and its mingled air of
commercial activity, physical wretchedness, and moral degradation.
It is from such a point of view that we receive the most forcible
illustration of those great changes whereof every country has been the
scene, and which are so tersely expressed by one who has gazed on the
revelations of geology with the eye of a true poet--
"There rolls the deep where grew the tree.
O earth, what changes hast thou seen!
There where the long street roars, bath been
The stillness of the central sea."
But the lifelessness of the carboniferous forests was amply compensated
by the activity that reigned in river, lagoon, and sea. Coral groves
gleamed white beneath the waves, fishes of many a shape disported in
stream and lake, and the bulkier forms, armed in massive plates of
bone, ascended the rivers or haunted the deeper recesses of the open
sea. In some beds of rock the remains of these various animals lie
crowded together like drifted tangle on the sea-shore, and the whole
reminds us of a vast cemetery or charnel-house. The bones lie at all
angles, many of them broken and disjointed as though the owner had died
at a distance, and his remains, sadly mutilated on the way, had been
borne to their last resting-place by the shifting currents; others
lie all in place, covered with their armature of scales, as though
the creature, conscious of approaching dissolution, had sought out a
sheltered nook and there lain down and died. It is not uninteresting
or uninstructive to tract; out in an old quarry stratum above stratum,
each with its groups of once living things. I know of few employments
more pleasant than to sit there, amid the calm stillness of a summer
evening, when the shadows are beginning to steal along the valleys and
creep up the hill-sides, and in that dim fading light to try in fancy
to clothe these dry bones with life, to picture the time when they
lived and moved in the glassy depths of lakes and seas, or amid the
solitudes of jungles and forests, and so to spend a pleasant hour in
reverie, till roused at last by the vesper song of the lark, or the low
meanings of the night wind as it sighs mournfully through the woods.
The study of fossil animals embraces a much greater range of subject
than that of fossil plants. The _fauna_ of any particular geological
formation, that is to say, its embedded animal remains, for the most
part vastly exceeds in number its _flora_, or vegetable remains, and is
likewise usually better preserved. About the nature and affinities of
several tribes of fossil plants there hangs an amount of uncertainty
which renders them a dubious guide to the climatal and other conditions
of the period and locality in which they lived. Generic distinctions
among living plants often rest on the character of those parts which
are the most perishable, such as flowers and seed-vessels. These
delicate structures we, of course, can hardly look to find preserved
in the rocks, and we have in place of them only detached leaflets,
twigs, branches, and stems, often sorely mutilated in outward form, and
presenting no trace of internal organization. But the tribes of the
animal kingdom have, for the most part, harder frameworks. The minute
infusoria, which by their accumulated remains help to choke up the
delta of the Nile, and swarm by millions in every ocean of the globe,
have their silicious or calcareous shells so minute that Ehrenberg
has estimated a cubic inch of tripoli to contain forty-one thousand
millions of them. The polypi have their internal calcareous skeletons,
which abound in all the older limestones, and form the coral reefs of
the present day. The mollusca, too, though, as their name imports, they
have perishable bodies, are yet, in most cases, furnished with hard
calcareous shells, that indicate by their various modifications of form
and structure, the character of the animal that lived within them. They
are found in all the formations from the earliest upwards, and as they
vastly exceed in numbers all the other classes with which the geologist
has to deal, they form the larger part of that basis of evidence from
which he interprets the past history of organized existence. Hugh
Miller loved to talk of them as the "shell alphabet," out of which the
language of palæontological history should be compiled. The vertebrata,
too, all have their hard skeletons, easily capable of preservation,
whether it be in the form of the massive exo-skeleton of bone that
characterized the older ganoidal fishes, or the compact endo-skeleton
of the reptiles and mammals. A greater amount of attention is,
therefore, due to the study of fossil animals, since they thus not
only far exceed fossil plants in number, but possess a higher value as
evidence of ancient physical conditions.
The _fauna_ of the Carboniferous system is a very numerous one,
exhibiting specimens of almost every class of animal life, from the
tiny _foraminifer_ up to the massive bone-covered sauroidal fish,
and even to occasional traces of true reptilian remains. By far the
larger number are peculiar to the sea, such as the molluscan tribes
and corals; others are undoubtedly terrestrial organisms, such as the
wings and wing-sheaths of several kinds of insects; while some appear
to be peculiar to fresh or brackish water, such as shells allied to
our _unio_ or river-mussel, and minute crustaceous animals known as
_cyprides_, of which we have still representatives in our ponds and
ditches. It is plain, then, that if we rightly ascertain the class or
family to which one of these fossils belonged, we shall obtain a clue
to the history of the physical geography, during Carboniferous times,
of the district in which the fossil occurs. A bed of unios will tell
us of old rivers and lakes that spread out their blue waters where
now, perchance, there lie waving fields of corn. A bed of corals and
stone-lilies will lay before us the bottom of an ancient ocean that
rolled its restless waves where to-day, perhaps, the quarryman plies
his task amid the gloom of dark pine-woods. In short, these organic
remains are to the history of the earth what ancient monuments are to
the history of man. They enable us to trace out the varied changes
of our planet and its inhabitants down to the human era, just as the
wooden canoe, the flint arrow-head, the stone coffin, the bronze sword,
the iron cuirass, the ruined abbey, and the feudal castle, teach us the
successive stages of progress in the history of our own country.
Whoever has spent a few days on some rocky coast, must have noticed
adhering to half-tide stones numerous solitary _actiniæ_. Arrayed in
all the colours of the rainbow--purple, green, and gold--these little
creatures hang out their tentacles like so many flowers, and have hence
received the popular name of sea-anemones. Their internal structure is
no less beautiful. They resemble so many large plump gooseberries, and
consist of a little sack suspended within a larger one. The outer sack
is fringed along its upper edges with one or more rows of slim hollow
tentacles, which diverge outwards like the petals of the daisy, and can
be contracted at pleasure so as somewhat to resemble the daisy when
folded up at sunset. The inner sack, which forms the stomach of the
animal, has a short opening or gullet, at the upper part of which is
the mouth lying in the centre of the cavity surrounded by the fringes
of tentacles. The inner sack is connected with the outer by means of
thin membranes, like so many partition-walls, which radiate inwards
like spokes towards the axle of a wheel. The space between each of
these membranes, or lamellæ, forms an independent chamber, but it has
a communication with those on either side by a window in each wall,
and further opens upwards into the hollow tentacles, which, with minute
orifices at their outer points, may be compared to chimneys. These
chambers form the breathing apparatus of the little creature. Sea-water
passes down through the tentacle into the hollow chamber below,
whence, by the constant action of minute hairlike cilia that line the
walls like tapestry, it is driven through the window into the next
chamber, thence into the next, and so on, passing gradually through the
tentacles back to the sea.
The actiniæ are of a soft perishable substance, but many of the other
_Anthozoa_, or flower-like animals, have hard calcareous skeletons. Of
such a kind are the polypi that in the Pacific Ocean have raised those
stupendous reefs and islands of coral. It does not appear that, during
the Carboniferous period, there existed any reef-building zoophytes,
but some of the most abundant forms of life belonged to a kindred
tribe, and are known by the name of _Cyathophyllidæ_, or cup-corals.
As the name imports, the typical genus has a general cup-shaped form,
but this is liable to many aberrations in the cognate genera. The
younger specimens of one species (_Cyathopsis fungites_) have a curved
outline somewhat like the bowl of a tobacco-pipe, whence the quarrymen
know them as pipe-heads. The older individuals are generally more or
less wrinkled and twisted, sometimes reaching a length of eight or nine
inches, and have been named by the workmen _rams'-horns_.
[Illustration: Fig. 16.--Cyathopsis (clisiophyllum ?) fungites.]
The annexed figure (Fig. 16) shows their general appearance and
structure. The lower end was fixed to the rock like the flat
sucker-like disc of the actinia. Around the outer margin there diverged
one or more rows of slim tentacles, hollow, soft, and retractile, like
those of the actinia. From the margin to the centre there radiated
more than a hundred lamellæ, but these differed from the corresponding
membranes of the modern animal, inasmuch as they were strengthened
internally by a skeleton of hard carbonate of lime; and to this
difference we owe their preservation. They stand out in high relief
upon weathered specimens, showing the long, narrow chambers that ran
between them. Their walls were once doubtless hung with countless
vibratile cilia, and perhaps pierced each with its window, through
which the currents of water passed in their ceaseless progress to and
from the sea. At the centre lay the mouth, communicating by a short
gullet with the stomach, which occupied the central portion of the
animal, and from the outer walls of which the lamellæ diverged like so
many buttresses. In its youngest stages, the animal occupied the whole
length of the cup, but, us it increased in size, it gradually retreated
from the narrow end, which was then divided off by a thin calcareous
membrane. At each successive stage of its growth, a new membrane was
added, each further and further from the lower end, so that eventually
the creature left below it a series of empty chambers all firmly
built up. Thus, in a specimen six or eight inches long, there would
in reality only be a small part tenanted--in fact merely the upper
floor--all the lower storeys remaining silent and uninhabited. The
house of this old-world architect differed widely in one respect from
human dwellings. Man begins his basement story of the same dimensions
as those that are to succeed it, or, if any difference is made at all,
the upper floors are built each less than the one below it, so that
the whole structure tapers upward to a point, as in the Pyramids. But
the cyathopsis reversed this latter process; it inverted the cone,
commencing the smallest chamber at the bottom, and placing the widest
at the top. Indeed, one is sometimes puzzled to conjecture how so bulky
a building could be securely poised on so narrow a basis, and it is
certainly difficult to see how the creature could move about with such
a ponderous load to drag along. The snail carries his house on his
back, yet it is a slim structure at the best; but the cup-coral must
not merely have carried his house, but some dozen or two of old ones
strung one after another to his tail. Perhaps, though free to move
about and try change of residence in its youthful days, the creature
gradually settled down in life, and took up its permanent abode in some
favourite retreat, the more especially as in process of time it became
what we should call a very respectable householder.
Allied to the cyathopsis is another and still more beautiful coral,
described so long ago as the latter part of the seventeenth century
by the Welsh antiquary and naturalist, Lhwyd, under the name of
_Lithostrotion_. Although many perfect specimens of it have been
found, and it is usually as well preserved as any of its congeners,
men of science have been sadly at a loss what to call it. Four or
five synonyms may be found applied to it in different works on
palæontology. There seems now, however, a tendency to return to the
name that old Lhwyd gave it two centuries ago; the family to which
it belongs, and of which it is the type, has accordingly been termed
the _Lithostrotionidæ_, and the species in question _Lithostrotion
striatum_ (Fig. 17). It differed from the cyathopsis in several
respects, but chiefly in this, that it lived in little congregated
groups or colonies, whereas the cyathopsis, like our own actinia, dwelt
alone.
[Illustration: Fig. 17.--Lithostrotion striatum.]
Each of these colonies was formed of a cluster of hexagonal, or rather
polygonal pillars, fitting closely into each other, like the basaltic
columns of Fingal's Cave, and springing from a common base at the sea
bottom.[30] Each pillar constituted the abode of a single animal,
and resembled generally the stalk of the cyathopsis. It had the same
minute diverging partitions running from the outer walls towards the
centre, and the same thin diaphragms, which, stretching horizontally
across the interior of the column at short intervals, marked the
successive stages of the animal's growth. Within these partitions,
which vary from forty to eighty in number, there runs an inner circular
tube with thin lamellæ and diaphragms. The exterior of the columns is
ribbed longitudinally by a set of long fine striæ, which give somewhat
the appearance of the fluting on a Corinthian pillar. The columns,
moreover, are not straight, but have an irregular, wrinkled outline,
so that, by a slant light, they look like some old pillar formed of
many layers of stone, the joints of which have wasted away, producing
an undulating profile in place of the original even one. But in these
ancient coral columns there is no blunted outline, no worn hollow;
the sculpturing stands out as sharp and fresh, and the wavy curves as
clearly defined, as though the creature had died but yesterday. They
resemble no order of human architecture, save faintly, perhaps, some of
the wavy outlines of the Arabesque.
[Footnote 30: Sir Roderick Murchison figures in his _Siluria_, p. 282,
a gigantic specimen, which measured two feet four inches in width.]
Despite all the improvements and inventions of modern times, classic
architecture has made no progress since the days of Pericles. All that
we do now is but to reproduce what the Greeks created 2000 years ago,
and he is reckoned the best architect who furnishes the best imitation.
Our architects might find some useful hints, however, by studying the
lowlier orders of nature. They would see there patterns of beauty far
more delicate than the Grecian capital, and more light and airy than
the Gothic shaft. And whether or not they could found a new order of
architecture, they could not fail to discover many modifications and
improvements upon some of the old. They could not readily light upon
a more graceful form than that of the lithostrotion, would they but
picture it as it grew at the bottom of the old carboniferous sea. A
group of hexagonal pillars, firmly compacted together like those of
the Giant's Causeway, or Fingal's Cave, rose from a white calcareous
pediment, as columns from the marble steps of an Athenian temple. Each
side of the pillar had a wavy undulating surface, delicately fluted by
long slender striæ, the whole being so arranged that the convexities
of one surface fitted into the sinuosities of the adhering one. Each
pillar was crowned above by a capital, consisting of the soft vibratile
tentacles of the animal, that hung over like so many acanthus leaves.
Of the form of these tentacles, their design and grouping, we know
nothing save what may be gathered from the analogy of living corals.
There can be little doubt, however, that, like the flower-shaped buds
of the existing reef-building polyps, they must have been eminently
beautiful, and in strict keeping with the graceful column which they
crowned.
Another kindred form was that known as the _lithodendron_. It, too,
grew in colonies, and seems to have closely resembled the last, save
that the pillars, in place of being six-sided, were round. I have seen
a bed of these corals several yards in extent, and seven or eight
inches deep, where the individuals were closely crowded together, so
as to resemble a series of tobacco-pipe stems, or slim pencils set on
end. The tubes, however, were not all quite straight; many being more
or less curved, and sometimes crossing their neighbours obliquely.
The internal arrangement was on the same plan as in the two previous
corals. The same numerous partitions ran from the exterior wall
towards the central tube, the same thick-set diaphragms crossed the
entire breadth of the column, imparting the same minute honey-combed
appearance to a cross section. The exterior of the column (in _L.
fasciculatum_) was likewise traversed by the same longitudinal striæ.
Both these corals seem to have been _fissiparous_, that is to say, they
propagated by splitting into two parts, each of which formed the base
of a new column with a new animal. The evidence for this statement
rests on the fact, that many of the tubes are seen to bifurcate in
their course, so that two new tubes are produced equal in size and
completeness to the old one from which they proceed. Another mode of
generation which, in at least its earlier stages, would produce a
somewhat similar appearance is called _gemmation_, and consists in
the protrusion of a bud or gemmule from the side of the animal, which
shortly develops into a new and perfect individual. It is probable,
however, that the ordinary mode of propagation among these old corals
was the usual one by impregnated ova. These ova, like those of our
sea-anemones, were probably generated within the partitions, between
the central stomach and the outer wall, whence they passed down into
the stomach, and were ejected by the mouth of the parent as little
gemmules, furnished with the power of locomotion by means of vibratile
cilia. Some of the _Medusa_ family possess this three-fold mode of
propagation; but, in all, the last-mentioned is the most usual.
Has the reader ever stretched himself along the shore, while, perhaps,
a July sun blazed overhead, and a fitful breeze came over the sea, just
strong enough to chase ashore an endless series of rippling wavelets,
and breathe over his temples a delicious and refreshing coolness?
Thus placed, and gazing dreamily now, perchance, at the distant sails
like white specks along the boundary line of sea and sky; now at the
gulls wheeling in broad circles through the air, and shooting swift
as arrows down into the blue water, he must often have turned to look
for a little at the sand which, heaped up in little mounds around him,
formed a couch well-nigh as soft as the finest down. Many a varied
fragment entered into the composition of that sand. Mingled among the
minuter quartzy particles lay scores of shells, some with the colour
not yet faded, and the valves still together--the delicate tellina,
with its polished surface, and its flush of pink; the cardium with its
strong white plaited sides, and the turritella with its circling spire;
some were worn down and sorely effaced, others broken into fragments
by the ceaseless grinding of the waves. It was pleasant labour in such
a sultry noon to pick out the shells of one species in all stages of
decay. The _Trochus lineatus_, or Silver Willie, as young ramblers by
the sea-shore love to call it, showed well the process of destruction.
The perfect shell, cast ashore, perhaps, by the last storm, and still
uninjured by the tides, displayed its russet epidermis, or outer skin,
covered with fine brown zig-zag lines, running across the whorls from
the creature's wide pearl-lined mouth to the apex. A second shell
exhibited a surface that had begun to suffer; the point had been
divested of its thin outer skin, and laid bare the silvery coating of
pearl below. A third had undergone a still longer period of abrasion,
for the whole of the epidermis was gone, and the surface gleamed with
a pearly iridescence. In yet a fourth, this bright exterior had been
in large measure worn away, and the blunted, rounded shell displayed
the dull white calcareous substance of which it was mainly built up.
But there were other objects of interest in the sand: bits of tangle,
crusted over with a fine net-work of gauze, and fragments of thin
leaf-like membrane, consisting of a similar slender network known
popularly as the _sea-mat_, occasionally turned up among the pebbles
and shells. No one who met with these organisms for the first time
could fail to be struck with the extreme delicacy of finish, if one
may so speak, that characterizes them. And yet he might be puzzled to
know what to make of them. The leaf-like membrane, at a first glance,
looks not unlike some of the flat-leaved algæ, and such the observer
might readily take them to be. Such, too, they were long regarded by
naturalists; but a more careful examination of them showed that the
so-called plants really belonged to the animal kingdom, and that the
supposed leaves were, in truth, the organic dwelling-places of minute
zoophytes, of which many hundreds lay grouped together on every square
inch. For many years these little creatures were called "celliferous
corallines," and classed among the polypi, that great tribe which has
its representatives in every ocean, from the coral reefs of the Pacific
to the little bell-shaped _hydra_ amid the tangle of our own seas.
But the microscope--that lamp which lights us into the inner recesses
of nature--revealed at last their true character. Fixed to one spot,
living in communities, and exceedingly minute, in short, with many of
the outward features of the true corallines, they were yet found to
possess a structure so complex and highly organized, as to entitle them
to rank among the higher tribes of the invertebrate animals, and they
are now accordingly pretty generally subjoined to the mollusca, under
the name of _Bryozoa_.
Each bryozoon consists externally of a single horny or calcareous cell,
sometimes furnished with a valve-like lid that folds down when the
animal withdraws itself. When danger is past, and the creature begins
again to emerge, the upper parts, which were drawn in like the inverted
finger of a glove, are pushed out until a series of tentacles, covered
with minute hair-like bodies, called cilia, are expanded. The vibratile
motion of these cilia causes a constant current in the direction of
the mouth, which lies in the centre of the hollow whence the tentacles
spring; animalcules are in this way brought in rapid succession within
reach of the mouth, and form a never-failing source of nourishment.
The interior is greatly more complex than that of the _polypi_. The
stomach is connected above with a cavity like the gizzard of a bird,
furnished with pointed sides, which serve to triturate the food before
it passes into the stomach. There is also a distinct intestine. The
muscular action for the expansion and retraction of the animal is
highly developed, and the generative system is a greatly more complex
one than that of the polyps already referred to. In short, however
closely they might be thought to resemble the corals in outward form,
their internal structure undoubtedly links them with a much higher type
of organization, and justifies the naturalist in subjoining them as a
sub-order to the mollusca.
The cells are grouped at short intervals along a horny or calcareous
substance, that sometimes encrusts sea-weed, or spreads out as a flat
leaf-like membrane, or rises into cup-shaped or dendritic forms. A
series of cells constituting a separate and independent colony, is
termed a polypidom. The cells are further connected together by an
external jelly-like integument, in which they are sunk, and which
serves to secrete the calcareous particles from the sea.
It is interesting to know that creatures so minute and yet so complexly
organized, existed abundantly in the seas of the Carboniferous period.
No less than fifty-four species are enumerated as having been obtained
from the carboniferous strata of the British Islands, and scarcely a
year passes without one or two new species being added to the list.
The most frequent belong to the genus _Fenestella_, or little window,
a name indicative of the reticulated grouping of the branches like the
wooden framework of a window. Each of these branches, or interstices,
as they are called, was more or less straight, being connected with
that on either side by a row of transverse bars, just as the central
mullion of an abbey window is connected with the flanking ones by means
of cross-bars of stone. Not unfrequently some of the branches subdivide
into two, as we saw to be the case among the cup-corals.
[Illustration: Fig. 18.--_a_, Fenestella oculata (M'Coy), nat. size;
_b_, magnified portion of the same.]
Fig. 18 illustrates the relative disposition of these branches. In _a_,
the natural size of the fossil is given; _b_ is a portion of the same
magnified, to show the form and arrangement of the ribs and cross-bars.
Each rib is seen to have two sides separated by a rounded ridge. Along
each side there runs a row of circular hollows or cells, every one
of which once formed the abode of a distinct bryozoon. The back or
inner surface of the branch, was ribbed and granulated irregularly,
without any cells. The connecting bars or dissepiments have no cells,
and served merely to bind the interstices together into one firm
organically-united polypidom. Such fragments as that here figured
are the most usual traces to be found of these animals among the
carboniferous rocks. But perfect specimens are sometimes met with which
show how delicate and graceful a structure the polypidom of some of the
fenestellæ must have been. All these bars sprung from a common point
as their basis, and rose up in the form of a cup. It was, in short, a
cup of network, hung with waving tentacles and quivering cilia. I have
seen some dissections of flowers in which all the softer tissue had
been removed, so as to present only the harder veinings of the leaves
with their thousand ramifications bleached to a delicate whiteness.
Out of these skeleton-leaves there were formed groups of lilies,
crocuses, geraniums, and roses, like patterns of the finest gauze. Some
of the larger-stemmed leaves that had been artistically moulded into a
tulip form, seemed not inaptly to represent the general contour of the
skeleton of the old carboniferous fenestella.
An allied form is called the _Retepora_. It differed from the previous
organism in having the ribs not straight, but irregularly anastomosing,
that is, running into and coalescing with each other, so as to form
a close network with oval interspaces, like a piece of very minute
wire-fence. Each of these wavy libs was completely covered over on one
side with oval pores or cells, which, as in the fenestella, formed the
abode of the living animals. The differences in organization between
the animal of fenestella and that of retepora can, of course, only be
matter of speculation. The general structure in both must, however,
have been pretty much alike. The former genus is now no longer extant,
but the latter, which was ushered into the world during the era of the
Old Red Sandstone, still lives in the deeper recesses of the ocean, and
manifests in its structure and habits the leading characteristics of
bryozoan life.
What rambler among old lime-quarries is not familiar with the
stone-lily, so abundant an organism in most of the Palæozoic and
many of the Secondary limestones? In some beds of the carboniferous
limestone its abundance is almost incredible. I have seen a weathered
cliff in which its remains stood out in bold relief, crowded together,
to use an expression of Dr. Buckland's, "as thickly as straws in a
corn-rick." The joints of this animal, known now as _entrochi_ or
wheel-stones, forced themselves on the notice of men during even the
middle ages, and an explanation was soon found for their existence.
From their occurring largely about the coast at Holy Island, they were
set down as the workmanship of Saint Cuthbert.
"On a rock by Lindisfarne,
St. Cuthbert sits and toils to frame
The sea-born beads which bear his name."
The aged saint was represented as employing his nights in this highly
intellectual task, sitting on a lone rock out in the sea, and using an
adjacent one as his anvil.
"Such tales had Whitby's fishers told,
And said they might his shape behold,
And hear his anvil sound,
A deaden'd clang,--a huge dim form
Seen but, and heard, when gathering storm
And night were closing round."
But these wheel-stones were not the only geological curiosities
to which this simple mode of explanation was applied. In the same
storied neighbourhood there occur in considerable numbers the round
whorled shells of the genus _Ammonites_. These were gravely set down
as petrified snakes wanting the head, and their petrifaction and
decapitation were alike reverently ascribed to the power of the sainted
abbess of Whitby.
"They told
How of a thousand snakes each one
Was changed into a coil of stone
When holy Hilda prayed."
The stone-lily belonged to that large class of animals ranked together
as _Echinodermata_, a name taken from one of the leading subdivisions
of the group--the _Echini_ or sea-urchins. It seems to have been one
of the earliest forms of life upon our planet, its disjointed stalks
occurring largely in some of the oldest Silurian limestones. In the
Secondary ages it began gradually to wane, until at the present day its
numerous genera appear to be represented by but the _comatula_ and the
_pentacrinite_, two tiny forms that float their jointed arms in the
profounder depths of the sea.
[Illustration: Fig. 19.--_a_, Cyathocrinites planus. _b_, Encrinal
stem, with uniform joints. _c_, Single joint, or wheelstone.]
As its name imports, the stone-lily or encrinite had a plant-like
form. It consisted of a long stalk fixed by the lower end to the
sea-bottom, and supporting above a lily-shaped cup, in which were
placed the mouth and stomach (Fig. 19 _a_). The stalk consisted of
circular plates (some of them not so thick as a sixpence), having
their flat sides covered with a set of minute ribs radiating from the
centre, and so arranged that the prominent lines of one joint fitted
into corresponding depressed lines of the adhering ones. The centre
of each joint was pierced by a small aperture, like the axle of a
wheel, which, when the stem was entire, formed part of the long tube
or canal that traversed the centre of the stem, and served to convey
aliment to the remotest part of the animal. Detached joints have thus
a wheel-like appearance (Fig 19 _c_), and hence their common name of
wheel-stones. In many species they were not all of the same diameter,
but alternately larger and smaller, as if the stem had been made up of
a tall pile of sixpences and threepenny pieces in alternate succession.
This variation gives a remarkably elegant contour to the stalk. The
flower-shaped cup consisted of a cavity formed of geometric calcareous
plates, and fringed along its upper margin with thick calcareous
arms, five or ten in number, that subdivided into still more slender
branches, which were fringed along their inner side with minute _cirri_
or feelers. All these subdivisions, however fine, were made up of
calcareous joints like the stalk, so that every stone-lily consisted
of many thousand pieces, each perfect in its organization and delicate
in its sculpturing. One species peculiar to the Liassic formation
(_Extracrinus Briareus_) has been calculated to contain one hundred and
fifty thousand joints!
The effect of this minute subdivision was to impart the most perfect
flexibility to even the smallest pinnule. The flower could instantly
collapse, and thus the animals on which the encrinite preyed were
seized and hurried to the central mouth. The lower part of the cup, or
_pelvis_, as it is called, contained the stomach and other viscera, and
communicated with the most distant part of the body by the central
alimentary canal.
But while this continued the general type on which the encrinites
were constructed, it received many minor modifications. These were
effected chiefly on the form and arrangement of the cup-shaped body
and its appendages, and form now the basis of our classification
into genera and species. Thus, in the genus known as _Platycrinus_,
the lower part of the cup consists of two rows of large hexagonal or
polygonal plates fitting closely into each other, while the upper part
rises into a dome-like elevation formed of smaller polygonal plates,
which have often a mammillated exterior. The arms sprang from the
widest part of the body where the large pieces of the lower cup were
succeeded by the small pieces of the upper. In an Irish species (_P.
triacontadactylus_), the arms subdivided into thirty branches, each
fringed with minuter pinnules and folding round the central elevated
spire, as the petals of a crocus close round its central pistil. In
another encrinite (_Poteriocrinites conicus_), the cup was shaped like
an inverted cone, the point being affixed to the summit of the stalk,
and the broad part throwing out from its edges the lateral arms. The
_Woodocrinus macrodactylus_ had such gigantic arms as well-nigh to
conceal the position of the cup, which relatively was very small in
size. They sprang from near the base of the cup, five in number, but
soon subdivided each into two, the ten arms thus produced being closely
fringed with the usual jointed calcareous pinnules.
The size and arrangement of the joints of the stalk also differed in
different genera. The Woodocrinus and many others had them alternately
broad and narrow, like a string of buttons of unequal sizes; others had
all the joints of the same relative diameter (Fig. 19 _b_), so that the
stalk tapered by a uniform line from base to point. I may add, that
on some specimens of both these kinds of stems, we can notice small,
solitary _areolæ_, or scars, which may mark the points of attachment
of cirri, or little tentacles, like those on the stem of the existing
Pentacrinite. But though each of these varieties of stem is peculiar to
a certain number of genera, there is often so little distinction among
the detached fragments, that it becomes difficult, indeed impossible,
to assign each to its appropriate individual. We may say, that certain
encrinal stalks could not have belonged to a poteriocrinus, and others
could never have fitted on to the cup of an actinocrinus; but we cannot
often say positively to what species they actually would have fitted.
There can, however, be no doubt about their being encrinites, and so
we have in them a safe and evident test for the origin of the rock in
which their remains occur. But to this I shall afterwards revert.
In the meantime, I would have the reader to fix the stone-lily in
his memory as peculiarly and emphatically a marine animal, dwelling
probably in the deeper and stiller recesses of the ocean, like the
Pentacrinite of existing times. Let him try to remember it, not in the
broken and sorely mutilated state in which we find it among the blocks
of our lime-quarries, but as it must have lived at the bottom of the
carboniferous seas. The oozy floor of these old waters lay thickly
covered with many a graceful production of the deep, submarine gardens
of
"Violet, asphodel, ivy, and vine-leaves, roses and lilies,
Coral and sea-fan, and tangle, the blooms and the palms of the ocean."
Amid this rich assemblage of animated forms, the stone-lilies must have
occupied a conspicuous place. Grouped in thick-set though diminutive
forests, these little creatures raised their waving stems, and spread
out their tremulous arms, like beds of tulips swaying in the evening
air. Their flower like cups, so delicately fringed, must have presented
a scene of ceaseless activity as they opened and closed, coiling up
while the animal seized its prey, or on the approach of danger, and
relaxing again when the food had been secured, or when the symptoms
of a coming enemy had passed away. Only from this animated action
would one have been apt to conjecture these organisms to be other
than vegetable. They lived, too, not in detached patches, like the
tulip-beds of the florist, but, to judge from the abundance of their
remains, must have covered acre after acre, and square mile 'after
square mile, with a dense growth of living, quivering flowers. As one
individual died out, another took its place, the decaying steins and
flowers meanwhile falling to pieces among the limy sediment that lay
thickly athwart the sea-bottom, and contributing, by their decay and
entombment, to build up those enormous masses of rock, known as the
mountain-limestone, which stretch through Yorkshire and the central
counties into Wales.
In addition to the stone-lilies, the carboniferous rocks contain the
remains of several other kinds of _Echinodermata_. Some of them find
their nearest modern analogues among the sea-urchins so common on
our shores; but I pass on to notice another very interesting class
of fossils known by the name of _Crustacea_, and still abundantly
represented, the crab and lobster being familiar examples.
The Crustacea, so called from the hard crust or shell which envelops
them, form, with all their orders and genera, a very numerous family.
They are of interest to us as containing among their number some of
the oldest forms of life. Away down in the lower Silurian rocks, among
the most ancient fossiliferous strata, we find the crustacean with
its armour of plates and its prominent sessile eyes set round with
lenses, still visible on the stone. Thus, on the first page of the
stony records of our planet's history are these primeval organisms
engraved. In some localities, where oxide of iron is largely present,
they are coated with a bright yellow efflorescence, and stand out
from the dull grey stone like figures embossed in gold.[31] On all
the subsequent leaves of this ancient chronicle, we can detect the
remains of crustacean life, and many tribes still swarm in our seas
and lakes. It is interesting, however, as marking the onward progress
of creation, to notice that, though this great family has continued
to live during all the successive geological ages, its members have
ever been changing, the older types waning and dying out, while newer
genera rose to supply for a time their place, and then passed away
before the advance of other and yet later forms. The trilobites that
meet us on the very verge of creation, swarmed by millions in the seas
of the Silurian ages, diminished gradually during the era of the Old
Red Sandstone, and seem to have died out altogether in the times of
the Coal. In no ocean of the present day is a trace of any of their
many genera to be seen. The _decapods_, of which our common crab is a
typical form, began to be after the trilobites had died out. In all the
subsequent eras they gradually increased in numbers, and at the present
day they form the most abundant order of crustacean life. The history
of these two divisions, to adopt Agassiz's mode of representation, may
be illustrated by two long tapering bands like two attenuated pyramids.
The one has its broad base resting upon the existing now, and thinning
away into the past, till at last it comes to a point. At a little
interval the apex of the other begins, and gradually swells outward as
it recedes, till the wide base terminates at the first beginnings of
life.
[Footnote 31: Such is the aspect of the organisms in some of the
Silurian sandstones near Girvan. I have seen the same bright tint
on a set of fossils from the Llandeilo flags of Wales, and from the
slates of Desertcreat, Ireland, and have disinterred similarly gilded
shells from the vertical greywackè slates of the Pentland Hills and
Peeblesshire. Nothing can be more beautiful than the aspect of these
fossils when first laid open, but the bright gleam eventually passes
away on exposure.]
But there are also some orders that would be best illustrated by a
long line of nearly uniform breadth, extending from the first geologic
periods to the present day. In other words, they seem to have retained
during all time pretty much the same amount of development. I shall
confine my notice of the carboniferous Crustacea to the description of
a single genus belonging to a family that seems to have begun during
the period of the Lower Silurian, and still flourishes abundantly in
existing waters.
[Illustration: Fig. 20.
1. Carboniferous cypris, nat. size, and magnified.
2. Recent cypris, highly magnified.
3. Carboniferous king-crab (_Limulus trilobitoides_).
]
The genus to which I refer is a well-known fossil in some parts of
the Coal-measure series, and has been named _Cypris_. The shells of
_cyprides_ are very minute, considerably less than the heads of small
pins (Fig. 20-1). They can be seen quite well, however, without the
use of a magnifying power. In shape they resemble beans, and when seen
scattered over a slab of shale, look much liker seeds than the relics
of animal life. Yet, under this simple exterior, they concealed a
somewhat complex organization. The little bean-shaped shells, which
are all that now remains to us of their structure, formed the crust
or outer shell in which their viscera were contained, and answered to
the massive carapace and segments of the crab. They consisted of two
valve-like cases fitting to each other, so as to resemble the united
valves of a bivalve shell. From the upper end there were protruded
through the opening between the valves a pair of slim jointed antennæ,
each furnished at its point with a bundle of minute hair-like cilia
(Fig. 20-2). These, when set in rapid motion, served to impel the
creature through the water. The legs, four in number, were encrusted
with the same hard membrane, and had the same jointed structure as
those of our common shrimps and crabs. The foremost pair were pointed
like the antennæ with fine hairs, the incessant action of which
assisted the animal in swimming. Of the little, confluent, sessile
eyes, the delicate branchia or gills, and all the complex internal
structure of the nervous, circulating, and other systems, no trace
has survived on the stone; but enough of the general external form is
left to show us the true affinities of these organisms in the animated
world of the present time. By studying the forms and habits of the
cyprides that swarm in some of our ponds and marshes, a just conception
is obtained of the structure and habitat of the animals that once
occupied the minute bean-shaped shells, which lie by millions among the
shales of the Carboniferous system. From such a comparison we infer,
that just as the cyprides of to-day are fresh-water animals abounding
among the green slime of stagnant pools, so, in past ages, they must
have preserved with the same organization the same habits. And thus
we arrive at the important conclusion that the strata in which the
remains of cyprides abound must have been deposited in lakes or rivers.
This gives us a key by which to interpret some of the changes of a
geological system, and the ancient physical revolutions of large tracts
of country.
The shales of the coal-measures sometimes contain the cypris cases in
such abundance as to derive therefrom a sort of fissile structure.
It should be borne in mind, however, that each animal may during its
lifetime have possessed in succession several of these cases. Among the
shell-bearing molluscous animals, the little shell which contains the
creature in its youngest stages remains ever after as an integral part
of the outer calcareous case. As the inhabitant grows, it continues
to add band after band to the outer edge of the shell, each of which,
whilst preserving the general symmetry and proportions of the whole
structure, increases its dimensions in every way. Among the univalves,
such, for example, as the turritella, so common on our shores, the
layers of growth succeed each other like the steps of one of those
long spiral stairs that our feudal forefathers loved to build from
the court-yard to the watch-tower of their castles. Each new layer
exceeding in bulk its predecessors, adds a new step to the ascending
pile, and thus the ever-widening mouth winds spirally upwards around
the central pillar. The bivalves exemplify the same principle. The
successive additions are made in a crescent form to the outer edges,
and form those prominent concentric ridges so conspicuous on many of
our commoner shells, such, for instance, as some of those in the genera
_Astarte_ and _Venus_.
But the architecture of the Crustacea (and, of course, that of the
cyprides) is conducted on a very different principle. Their houses
admit of no additions or enlargements, and so, when the animals find
themselves getting somewhat straitened, they retire to a sheltered
spot, and there, separating the walls that hem them in, crawl out like
soft lumps of dough. The outer membrane of the moulted animal quickly
acquires strength and hardness, and in a day or two the renovated
creature is as healthy and vigorous as ever. In this process it is
not merely an external shell, like that of a mollusc, which is thrown
off, but a veritable skin, so that when the old shell is abandoned
it frequently could not be detected on a first glance to be empty,
the outer crust of every leg and joint, and sometimes even of thin
bristles, remaining just as in the living animal.
It is not unlikely that this process of moulting takes place annually
in most of the Crustacea, so that if we suppose a fossil member of the
group to have lived six years, it would have left six crusts to be
entombed in any deposits that might be forming at the time. Of course
there would be many chances against all the six being preserved, but
the possibility of at least several of them becoming fossilized should
be borne in mind when we speculate on the abundance of such organisms
in any geological formation.
I might refer to another very interesting group of crustacean animals
known as the _Limuli_, or king-crabs, of which there were at least
three representatives during the times of the English Carboniferous
system (Fig. 20-3). They are remarkable chiefly for their large
crescent-shaped shield, their long sword-like tail, and their double
pair of eyes, of which the outer ones are large, sessile, and compound,
like those of the trilobites, while the middle pair are small, simple,
and set close together on the forehead, like those of the single-eyed
Cyclops in the old mythology. Altogether, with their shields, swords,
watchful waking eyes, strong massive armour, and great size (for some
of them measure two feet in length), they form a most warlike genus.
CHAPTER VI.
Carboniferous fauna continued--George Herbert's ode on
"Man"--His idea of creation--What nature teaches on
this subject--Molluscous animals--Range of species in
time proportionate to their distribution in space--Two
principles of renovation and decay exhibited alike in the
physical world and the world of life--Their effects--The
mollusca--Abundantly represented in the carboniferous
rocks--Pteropods--Brachiopods--Productus--Its alliance with
Spirifer--Spirifer--Terebratula--Lamellibranchs--Gastropods--
Land-snail of Nova Scotia--Cephalopods--Structure of orthoceras--
Habits of living nautilus.
Holy George Herbert, in one of the most remarkable odes of the
seventeenth century, sang quaintly, yet nobly, of the dignity of man.
He looked into the design and nature of the human heart, and saw there
a palace that had been built for the abode of the Eternal. Deserted
though it might be, broken down and in ruins, yet there still lingered
a trace of its ancient glory, and the whole material world still
testified to its inherent greatness. He looked abroad on the face of
nature, and saw, in all its objects and all its movements, a continued
ministration to man.
"For us the windes do blow;
The earth doth rest, heav'n move, and fountains flow.
Nothing we see, but means our good,
As our delight, or as our treasure;
The whole is, either our cupboard of food,
Or cabinet of pleasure.
"The starres have us to bed;
Night draws the curtain, which the sunne withdraws;
Musick and light attend our head.
All things unto our flesh are kinde
In their descent and being; to our minde
In their ascent and cause."
The idea is a very natural one, and is consequently as old as man
himself. Human vanity is soothed by the reflection that all this varied
world, with its countless beauties, has been designed and arrayed
solely for the use of man. And yet, if we but think of it, such a view
of creation, however natural and pleasing, is at the best but a narrow
and selfish one. It assuredly finds no response in nature, and grows
more and more out of fashion the further our investigations proceed.
Nature teaches us that long ere man appeared upon the earth there were
successive generations of living things just as now; that the sun
shone, and the waves rolled, and the wind blew, as they do to-day; and
that, on as lovely a planet as that whereon we dwell, there lay forests
and prairies nursing in abundance animals of long-extinct forms; lakes
and rivers, haunted by creatures that find no representatives now;
and seas teeming with life, from the minute infusory up to the most
unwieldy icthyosaur, or the most gigantic cetacean. And all this, too,
ere a reasoning, intelligent being had been numbered among terrestrial
creatures, and when, perhaps, each successive creation was witnessed
by none save those "morning stars who sang together, and those sons
of God who shouted for joy." The delight and comfort of the human
race formed, doubtless, one of the many reasons why this globe was so
bountifully garnished.[32] But the workmanship of a Being infinitely
wise, and good, and powerful, could hardly have been other than
complex and beautiful. That symmetry and grace which we see running
as a silver thread through every part of creation, forms one of the
characteristics of the Almighty's mode of working. From the Fountain
of all Beauty nothing unseemly or deformed can proceed. And so we
find, away back among the ages of the past, that, though the material
world might be less complete, it was not less beautiful than now. Nay,
those bygone millenniums stood higher in one respect, for the eye of
God rested upon their unsullied glory, and he pronounced them very
good; but these last ages of creation are dimmed and darkened, and
that Eye now watches a world trodden down by the powers of evil. There
is profound truth in the sublime allegory of Milton that represents
Sin girt round with clamorous hell-hounds, and the two grisly forms
sitting at the farthest verge of purity and light, to keep the gates of
darkness and chaos. With the introduction of moral evil into our planet
came the elements of deformity and confusion. The geologist can go back
to a time ere yet the harmony of nature had been broken. The Christian
looks forward to a day when that harmony shall be again restored, and
when guilt with all its hideous train shall be for ever chased away
from the abodes of the redeemed.
[Footnote 32: In connexion with this subject I have been often struck
with a passage in St. Paul's Epistle to the Colossians, i. 10, "All
things were created by him [Christ] and for [εις--with a view to,
on account of] him." It is probable that these words, in their full
meaning, cannot be understood by us. Yet they seem to point to Christ
as at once the Creator, and himself the acme and design of creation;
and perhaps they may contain what hereafter shall prove the key to
the mystery of creation. On this impressive and difficult subject the
reader should refer to the closing chapter of Hugh Miller's _Footprints
of the Creator_. See also M'Cosh on _Typical Forms_, 2d edit. p. 531.]
Such thoughts as these sometimes arise in the mind of one who labours
much among organic remains. By no class of fossils are they more
vividly suggested than by those which we come next to examine--the
various tribes of molluscous animals. This results from the high
antiquity of these organisms, and the similarity of type which they
have manifested in all ages. In the very earliest geological periods
they exhibited the same symmetry of external form as now, the same
beauty of structure, and apparently the same delicacy of colour. Nay,
so closely did they resemble their existing congeners that we are
seldom at a loss as to their affinities, and can refer them to their
places in the scale of creation, and sometimes even to genera still
living.[33]
[Footnote 33: It must be admitted, however, that not a few of the
identifications already made are somewhat suspicious The natural
tendency is to perceive resemblances--a tendency which even the most
rigid science sometimes fails to control.]
The geological ages saw many strange types of creation. One era, in
especial, furnished reptiles which united in their structure the snout
of the porpoise, the head of the lizard, the teeth of the crocodile,
the paddles of the whale, and the backbone of the fish. Some displayed
the long pliant neck of the swan, and others careered through the
air on wings like those of the bat. But the molluscous tribes have
never exhibited such aberrant forms. The existing classes and orders
of the naturalist are still the same as those which nourished during
the successive geological periods. Hence their value as evidence of
physical changes in the ancient world. Hence, too, the conviction,
forced upon the mind of the observer, that the conditions for the
support of life never deviated much from those now in operation; that
in place of all the varied beauty of the world having arisen for the
use of man, it existed millions of years ere the breath of life had
been breathed into his nostrils; that in fine, man is but a new-comer,
a creation of yesterday.
There is another point suggested by the occurrence of mollusca in the
Carboniferous system, to which it may be well to refer, namely, the
curious, and as yet not wholly understood fact, that the range of
animals in time is in some way proportionate to their range in space.
In other words, it often happens (so often, indeed, as apparently to
indicate a law) that the more widely diffused a genus is found to be
at the present day, the farther back can we trace its remains into the
geological ages. This fact probably depends upon causes, many of which
are still unknown to us; but the following remarks may help the reader
to a notion of the general bearings of the subject.[34]
[Footnote 34: The law is more especially exemplified by the mollusca,
but it may eventually be found to characterize other classes. We,
perhaps, see traces of it in the present distribution of the two most
ancient orders of icthyic life--the placoids and ganoids.]
In the profounder recesses of the ocean, the temperature remains more
or less uniform all over the globe.[35] In these undisturbed regions
there occur, along with corals and other humble animals, many kinds of
mollusca, such as terebratulæ, craniæ, scissurellæ, &c. These are very
generally found not to be confined to one province or limited district,
but to flourish in every sea from Hudson's Bay to Hindustan. One of
the causes of this wide distribution is the uniformity of temperature
that characterizes the depths in which they live. They can migrate
from one ocean to another, from the torrid zone to the polar circle,
without experiencing any destructive change in the thermal conditions
of their element. And provided only they meet with no barrier in the
form of a lofty submarine mountain chain or profound abyss, and can
secure the requisite food in their journey, we know no reason why some
of these shells may not thus extend themselves over wide areas. Of the
two species of _rhynconella_ now living, one inhabits the depths of
the icy sea, the other enjoys the warmer waters that lave New Zealand.
The species, in this case, seem (for the fact cannot yet be accepted
as fully proved) to occupy a more limited area, while the genus has a
larger range.
[Footnote 35: The stratum of constant temperature runs in a wave-like
form from pole to pole. In the arctic and antarctic oceans it is found
at a depth of 4500 feet, whence it slopes upwards so as to reach the
surf ice at the temperate zone on both sides of the equator. It then
gradually sinks down in the warmer regions, till at the equator it is
7200 feet below the sea-level. There are thus one tropical and two
polar basins separated by two wave-like circles, or, as a geologist
would say, three synclinal troughs separated by two anticlinal ridges.]
Now, a genus widely diffused, and capable of enduring great differences
in the temperature and other conditions of the ocean, would probably
suffer least from any great physical changes. If all the sea at one
locality were converted into land, the genus would be driven into other
districts, and thrive as abundantly as ever; or, even supposing that it
should become locally extinct, it would still be abundantly represented
in other oceans of the globe. In the course of many ages, after many
such slow revolutions in the configuration of land and sea, the genus
might perhaps become greatly reduced in numbers, until at length some
final elevation of the sea-bed, or other change, might cause its total
extinction. In the _rhynconella_, we perhaps see one of these genera in
its last stage. Any great change in northern latitudes would probably
destroy the arctic species, and a similar change around New Zealand
might gradually extinguish the southern one.
Looking, then, from this point of view into the past history of life
upon our planet, we see that such extinctions have often taken place.
At first, many of these widely-diffused genera were created. They
were represented by a large number of species as well as individuals,
and ranged over all the oceans of the globe; but in tracing out their
history, we mark one species after another passing away. Some of them
lived for but a comparatively short period; others came in with the
beginning and saw out the end of an entire geological system; but of
all these early species there is not now a single one extant, though
some of the genera still inhabit our seas. It is plain, therefore,
that but for the operation of another principle, all the genera, too,
would ere this have become extinct, for the whole can contain no more
than the sum of its parts; and if these parts are destroyed the whole
must perish simultaneously. As the species of certain genera died out,
however, their places were from time to time filled up with new ones,
yet the rate of increase became ever less and less than the rate of
decrease, so that the numbers of such genera grew fewer with every
successive period, and have reached their minimum in existing seas.
There are instances, however, in which this ratio was reversed, the
list of added species continually outnumbering that of the extinct,
till the genus reached its maximum, when it either continued at that
stage till the present day, or began slowly to decline.
In the physical world around us, we behold a perpetual strife between
the two great principles of renovation and decay. Hills are insensibly
crumbling into valleys; valleys are gradually cut down, and their
debris transported to the sea. Our shores bear witness to the slow
but ever onward march of the ocean, whether as shattered cliff's worn
by the incessant lashing of the surge, or as sand-banks and submerged
forests that represent the wolds and holms of our forefathers. We mark,
too, how the sediment thus borne into the main is sowing
"The dust of continents to be;"
while the slow elevation of large tracts of country, or the sudden
upheaval of others, shows us by how powerful an agency the balance of
land and sea is preserved, and how sometimes the paroxysm of an hour
may effect a mightier change than the wasting and decay of a thousand
years. We choose to call these two principles antagonistic, because in
their effects they are entirely opposite; yet there is no discordance,
no caprice in their operation. Each works out its end, and the result
is the harmony and stability of the face of nature.
In the world of life, too, there seems to have been a double principle
of decline and renewal. The natural tendency of species and genera,
like that of individuals, has been towards extinction. Why it should be
so we know not, further than that they are for the most part influenced
by every change in physical geography. But they probably obey a still
higher law which governs their duration, as the laws of vitality govern
the life of an individual If we are but slightly acquainted with the
agency by which the degradation of land is counter-balanced, we are
still more ignorant of the laws that preserve the balance of life.
Creation is a mystery, and such it must for ever remain. So, too, are
the principles on which it has been conducted. We can but mark their
results. We see new species appear from time to time in the upward
series of the geological formations, but they tell not whence they
came. Of two genera created together at the beginning, one ere long
died out, but the other still lives; yet here there is assuredly
nought like discordance or caprice. Nay, these two principles--death
and creation--have been in active operation all through the ages, and
the result is that varied and exquisitely beautiful world wherein we
dwell.
The Mollusca are so named from the soft nature of their bodies, and are
familiar to us as exemplified in the garden-snail and the shells of the
sea-shore. The general type upon which they are constructed is that of
an external muscular bag, either entire or divided into two, called
the mantle, in which the viscera are contained. In most of the orders,
they have likewise an outer hard calcareous shell, consisting of one or
more parts. It is of course this shell alone that can be detected in
the rocks, but by attending to the relations between the living animals
and their shells, we ascertain the nature and affinities of the fossil
species.
Few who ramble by the sea-shore, gathering limpets, whelks, and
cockles, are aware how complex an anatomy is concealed within one
of those brown discoloured shells. There are elaborate nervous and
muscular systems--sometimes several hearts with accompanying arteries
and veins--often dozens of rudimentary eyes--capsules which perform
the function of ears--jaws, teeth, a strongly armed tongue--gullet,
gizzard, stomach, liver, intestine, and complete breathing apparatus.
The structure and grouping of these parts vary in the different genera
and orders, and upon such variations is founded the classification of
the naturalist. Thus, the mollusca of the highest class are called
the _Cephalopoda_, or _head-footed_, because their feet, or rather
arms, are slung in a belt round the head. They contain, among their
number, the cuttle-fish, with its curious internal bone that shadows
forth, as it were, the coming of the vertebrate type; and the nautilus,
with its many-decked vessel of pearl. The second class is termed the
_Gastropoda_, or _belly-footed_, as the genera embraced under it
creep on the under side of the body, which is expanded into a broad
retractile foot. The common snail and whelk are familiar examples. The
third class is formed by the _Pteropoda_, or _wing-footed_--delicate
animals, found only in the open sea, and remarkable for a pair
of wing-like expansions or fins on the sides of the mouth. The
_Lamellibranchiata_ form the fourth class, and receive their name
from the laminated form of their branchia, or gills. They contain the
two-valved shells, such as the oyster and scallop, and are one of the
most abundant groups of animals on our coasts. The fifth class consists
of the _Brachiopoda_, or _arm-footed_ molluscs a name given to them
from their long spiral arms, once thought to be the instruments of
motion, but now ascertained only to assist in bringing the food to the
mouth. The sixth, and humblest class, has received the designation of
_Tunicata_, from the thick bladder-like tunic, or sac, which supplies
the place of an outer shell.
The geologist finds the remains of all these classes in the different
rock-formations of the crust of the earth. They flourished so
abundantly in the earliest seas, that the first geological period
has sometimes been called the Age of Molluscs; and, during all the
subsequent eras, they held a prominent place among the inhabitants of
the deep. Let us look for a little at their development in the times of
the Carboniferous system.
As the Carboniferous group of rocks exhibits the remains of ocean-bed,
lake-bottom, and land-surface, so we find in it shells of marine,
fresh-water and (though rarely) terrestrial mollusca. The marine
genera greatly predominate, just as the shells of the sea at the
present day vastly outnumber those either of lakes or of the land. In
England they occur chiefly in the lower part of the formation, giving a
characteristic stamp to the deep series of beds known as the mountain
limestone. There they are associated with the corals and stone-lilies
already described--all productions of the sea. In Northumberland,
however, and generally throughout Scotland, they occupy a somewhat
different position. The great mountain limestone of central England
gets split up into subdivisions as it proceeds northward, and beds of
coal, full of land plants, become mingled with the ordinary marine
strata. Sometimes we may find a group of brachiopods scattered over the
macerated stem of a stigmaria; and the writer has himself collected a
sigillaria in a limestone crowded with stone-lilies and _producti_.
But this intermingling is still further carried on in the upper part
of the series. The coal-beds, with their underclays and stigmaria
rootlets, evidently representing ancient vegetation with the soils
on which it grew, are succeeded by beds of limestone, full of marine
mollusca; and these, again, are erelong replaced by sandstones,
shales, and ironstones, charged with land-plants and fresh-water
shells. To this curious blending of very different organic remains, I
shall have occasion to refer more at large in a subsequent chapter. I
mention it now as a sort of apology for the dryness of details which
it is necessary to give, in order to complete our picture of the
carboniferous fauna, and to understand the principles upon which the
ancient history of the earth is deciphered.
[Illustration: Fig. 21.]
Of the _Pteropoda_, we have, as yet, but one carboniferous genus,
the _conularia_ (Fig. 21). It was a slim delicate shell, in shape an
oblong cone, having four sides, finely striated with a sort of zig-zag
moulding like that of the Norman arch. Each of the four angles was
traversed along its whole extent by a narrow gutter-like depression,
and this style of fluting, combined with the markings on the sides,
imparted no little elegance to the shell. The conularia is not a
common fossil. It has been found among the coal-bearing strata of
Coalbrook-Dale, and was noticed long ago by Dr. Ure in his _History of
Rutherglen_.
The _Brachiopoda_ are bivalve molluscs, but unlike most other molluscs
they are rooted to one spot, and destitute of any power of locomotion.
Their shells are unequal, the dorsal, or upper valve, being smaller
and usually more bulged out than the under or ventral valve, which
in most species is prolonged at its narrow end into a kind of beak.
In the terebratula this beak has a little circular hole, from which
there emerges a short peduncle or stalk, that fixes itself firmly to a
rock or other substance at the sea-bottom, and serves the purpose of
an anchor and cable to keep the little vessel safely moored. When the
shells are detached, these perforated ventral valves have so exactly
the form of the old Roman lamps, "that they were called _Lampades_,
or lamp-shells, by the old naturalists."[36] Other species, as the
_lingulæ_, have no beak, and the long peduncle passes out between the
valves, which are of nearly equal size, and have been compared to the
shape of a duck's bill. In yet another genus, the _crania_, there is
no peduncle, but the animal adheres by its lower valve, much like
the oyster, and may often be seen clustered in groups on decayed sea
urchins or other organisms, particularly in the chalk formation.
[Footnote 36: See the excellent _Manual of Mollusca_, by Woodward, p.
209.]
The internal structure of these animals is singularly beautiful. The
inner surface of each valve is lined with a soft membranous substance,
called the pallial lobe, the margin of which is set round with stiff
hair-like bristles, that prevent the ingress of any foreign body likely
to interfere with the play of the delicate filaments of the arms. These
two soft lobes are furnished with veins, and supply the place of a
breathing apparatus. The body of the animal occupies not quite a third
part of the interior of its valves, and is situated at the narrow end.
There are thus two distinct regions within the shell, separated from
each other by a strong membrane, through the centre of which is the
opening of the mouth. The smaller cavity next the hinge contains the
viscera, and the outer larger one, the folded and ciliated arms. These
arms form one of the most characteristic features of the brachiopods.
They are two in number, and proceeding from the margin of the mouth,
advance into the outer empty chamber of the shell, and return upon
themselves in spiral curves and folds. They are fringed with slim,
flat, narrow filaments, set along the arm like teeth along the back of
a fine comb. Though called arms, these long ciliated appendages are
rather enormously protruded lips. The vibratory action of the fringes
causes currents to set inwards towards the mouth, which is placed at
the inner end or base of the arms. To support these long convoluted
arms, many of the genera are furnished with slender hoops of hard
calcareous matter, which are hung from the dorsal valve, and are still
found within the shells of some of the most ancient fossil brachiopods.
The little visceral cavity contains the complex groups of muscles for
opening and closing the valves, a simple stomach, a large granular
liver, a short intestine, two hearts, and the centre of the nervous
system. Without going into the details of these various structures,
the reader will see that the brachiopoda are really a highly organized
tribe; and I am thus particular in the enumeration, partly that he may
the better understand the mechanism of the carboniferous shells of
that type, and partly that he may mark how the oldest forms of life,
those that meet us on the very threshold of animated existence, were
not low in organization, but possessed an anatomy as complex as it was
beautiful.
Who that has ever wielded an enthusiastic hammer among the richly
fossiliferous beds of the mountain limestone, does not remember with
delight the hosts of delicately fluted shells that the labour of an
hour could pile up before him? There was the striated productus,
with its slim spines scattered over the stone. There, too, lay the
spirifer with its broader plications, its toothed margin, and its
deeply indented valve. Less common, and so more highly prized, was the
slimly-ribbed rhynconella, with its sharp, prominent beak, or perhaps
the smooth, thin terebratula, with its colour-bands not yet effaced.
These were pleasant hours, and their memory must dwell gratefully
among the recollections of one whose avocations immure him throughout
well-nigh the livelong year amid the din and dust of town--the _fumum
et opes strepitumque Romæ_.
[Illustration: Fig. 22.--Productus giganteus.]
In the _productus_ the dorsal valve is sometimes quite flat, while
the ventral is prominently arched, and the shell resembles a little
cup with a flat plate of the same diameter placed over it. Usually,
however, both the valves are concavo-convex, or arched in the same
direction like two saucers placed within each other. The exterior
surface of each valve is differently ornamented in the various species.
A very common style of sculpturing is by a set of fine hair-like
longitudinal ribs, diverging more or less regularly from the hinge line
to the outer margin. In some species these ribs are wider, and are
furnished with little prominent scars. In others (as _P. punctatus_)
a set of semicircular ridges runs round the shell, narrowing as they
converge from the outer lips to the centre of the hinge line, and
bearing each an irregular row of small scars or tubercules. Some of
the species are very irregularly ornamented into a sort of wrinkled
surface, in which the striæ seem, as it were, thrown over the valves in
bundles at random.
The productus was furnished with slender hollow spines, which rose up
from the surface of either valve, chiefly, however, about the hinge. In
_P. spinosus_ they were long and stout, like thin rush stalks, while in
the smaller species they rather resembled stiff bristles. The use of
these spines is not very well made out. As most of the producti appear
to have been free, that is, without any peduncle fixing them to the
sea-bottom, it has been conjectured that the spines, by sinking deep
into the mud, may have served the place of a peduncle to moor the shell.
As regards size, the productus is very variable. You may gather some
species in the young form, not larger than peas, while others may
reward your search, having a breadth of six or eight inches (_P.
giganteus_). But however much they may vary in dimensions, they usually
remain pretty constant in their abundance, being among the most common
fossils of the mountain limestone, and even of some limestones in the
true Coal-measures;[37] and that must be a poor stratum indeed which
cannot yield you a bagful of producti.
[Footnote 37: See the table given below in Chap. X.]
The productus no longer ranks among living forms. It began during the
times of the Upper Silurian system, lived all through the Old Red
Sandstone, and attained its maximum of development in the seas of the
Lower Carboniferous group. As the coal forests began to flourish, the
productus seems to have waned; but it is still sometimes found in
considerable numbers in the ironstones and limestones intercalated
among the coal seams of northern England and central Scotland. In the
period which succeeded the coal, that, namely, of the Permian, it seems
to have died out altogether, at least no trace of its remains have as
yet been detected in strata of a later age. But whilst it lived, the
productus must have enjoyed a wide range of climate, for its valves
have been found by thousands both in the old world and in the new. I
have seen several that were brought from the hills of China, and they
occur likewise in Thibet. Specimens have been brought, too, from the
warm plains of Australia, and from the snows of Spitzbergen.
In looking over the fossils that lie grouped along beds of the
mountain limestone, there are two forms that we find almost invariably
side by side--the productus and the spirifer. They seem to have
begun life together, or rather, perhaps, the spirifer is somewhat
the older brother. They voyaged through the same seas, and anchored
themselves to the same ocean-bed, sometimes among mud and ooze, and
often among bowers of corals and stone-lilies. They visited together
the most distant parts of the world, from China to Chili, and from
Hudson's Bay to New Zealand. I have sometimes laid open fragments of
limestone where they lay thickly clustered as though they had ended
a life of friendship by dying very lovingly together. But after all
the varieties of the productus had died out, some species of the
spirifer still lived on, and it was not until the period of the lias
that they finally disappeared. I remember meeting with one of these
latest spirifers in the course of a ramble in early morning along the
shores of Pabba, one of the lone sea-girt islands of the Hebrides,
where the Scottish secondary rocks are represented. The beach was
formed of low shelving reefs of a dark-brown micaceous shale, richly
charged with the characteristic fossils of the Lias--ammonites,
belemnites, gryphææ, pectines, &c. In the course of the walk I came to
a lighter coloured band, with many reddish-brown nodules of ironstone,
but with no observable fossils. A search, however, of a few minutes
disclosed a weathered specimen, near which a limpet had made good its
resting-place; and this solitary specimen proved to be one of the last
lingering spirifers (_S. Walcottii_). The form struck me at once as a
familiar one, and recalled the fossils of the mountain limestone. It
may seem a puerile fancy, but to one who had lately been working among
palæozoic rocks, and remembered the history of the spirifer, there
was something suggestive in the loneliness of the specimen. With the
exception of one or two other organisms (as _rhynconella_), it was by
far the most ancient form of the deposit. Its family had come into the
world thousands of years before that of the large pinnæ that lay among
the neighbouring shales, and perhaps millions of years before that of
the gracefully curved ammonites. But the family was nearly extinct
when these shales were being thrown down as sandy mud, and this wasted
specimen, worn by the dash of the waves, seemed in its solitariness no
inapt representative of an ancient genus that was passing away.
The spirifer received its name from the two highly developed spiral
processes in the interior of the shell attached to the dorsal valve.
They were hard, like the substance of the shell, and sprang from
near the hinge, each diverging outwards to near the border of the
valve. They resembled two cork-screws, but the loops were much closer
together. These coiled calcareous wires almost filled the hollow of the
shell (Fig. 23), and ample support was thus afforded to the filamentous
arms. In recent brachiopods, these arms do not always strictly follow
the course of the calcareous loops. Among palæozoic genera the case may
have been similar, so that the complex calcareous coil of the spirifer
may not perhaps indicate a corresponding complexity of the arms. But
none of the few recent forms exhibit anything like the coiled processes
of the spirifer.
The Carboniferous system of Great Britain and Ireland is stated to
have yielded between fifty and sixty species of spirifers. Of course,
in such a long list the gradations are sometimes very nice, and to an
ordinary eye imperceptible, but there exist many marked differences
notwithstanding. The general type of the spirifers is tolerably well
defined. They had both valves arched outwards, not concavo-convex as
in the productus. Their hinge-line, like that of the latter shell,
ran in a straight line, and their dorsal valve was raised along its
centre from hinge to outer margin, into a prominent ridge, while in the
ventral valve there was a furrow exactly to correspond. Most of the
species were traversed by sharp ribs radiating from the centre of the
hinge-line like those on the surface of the common cockle. But some
were quite smooth, retaining only the high lobe in the centre, such as
_S. glaber_. In a noble specimen figured by M'Coy[38] under the name
of _S. princeps_, the valves are covered with broad plaits that sweep
gracefully outward from the centre of the hinge-line.
[Footnote 38: _Carb. Limest. Foss. of Ireland_, pl. 21, fig. 1.]
[Illustration: Fig. 23.--Spirifer hystericus. _b_, Interior of the
same, showing the arrangement of the spiral arms.]
The spirifers vary more in form than in external ornament. Some are
triangular, others nearly semicircular, others long and attenuated. In
some species (as the _S. glaber_), the central ridge is very prominent,
taking up about a third of the entire area of the shell, and thus
giving it a trilobed appearance. In others (as _S. symmetricus_) it is
less marked, and bears a minor furrow down its centre; while in yet a
third class (as in some specimens of _S. trigonalis_) the median fold
scarcely rises above the ribs that are ranged on each side.
These old shells probably anchored themselves to the sea-bottom by
means of a thin peduncle, and lived by the vigorous action of those
complex fringed arms, whose screw-like skeleton still occasionally
remains, and which conveyed to the mouth the animal substances that
served as food.
[Illustration: Fig. 24.--Terebratula hastata.]
I shall refer to but one other brachiopod of the carboniferous rocks,
interesting both as one of the forms of life still living in our
seas, and as exhibiting, after the lapse of such a vast interval, the
form of the coloured bands which adorned it when alive. It is called
_Terebratula hastata_; a slim delicate shell like its representatives
of the present day, narrow at the beak, and bulging out towards the
outer margin, which is slightly curved. The surface is smooth, and
in the older specimens has numerous concentric layers of growth,
especially marked near the margin. The stripes of colour radiate from
the beak, outwards, and though the tint which once brightened them is
no longer visible, it may be that the vessel of the little terebratula,
which lay anchored perhaps fifty fathoms down, was well-nigh as
gaily decked as a felucca of the Levant. But the existence of these
colour-bands is not merely interesting; the geologist can turn it
to account in investigating the physical conditions of an ancient
ocean. The late Professor Edward Forbes, after a careful series of
investigations in the Mediterranean, brought to light the fact, that
below a depth of fifty fathoms shells are but dimly coloured, and hence
he inferred, from the numerous coloured shells of the carboniferous
limestone, that the ocean in which they lived was not much more than
fifty fathoms deep.[39]
[Footnote 39: Similar coloured bands are found even in the Lower
Silurian, e.g., on turbo rupestris (Murchison's _Siluria_, p. 194), while
on many of the carboniferous gastropods and lamellibranchiate bivalves,
they are of frequent occurrence.]
The _lamellibranchiate_ bivalve shells of the British Carboniferous
system, so far as yet discovered, number about 300 species, belonging
to genera some of which are still familiar to us. There were the
_pectens_ or _scallops_, the _pinnas_ with their beards of byssus,
the _cardiums_ or cockles, the _mytili_ and _modiolæ_ or mussels,
all sea-shells. Then among the fresh-water bivalves we can detect
several species of the unio or river mussel, that perhaps displayed
valves as silvery in their lining as those of our own pearl-mussels.
But with these well-known forms there co-existed some that no longer
survive. Such was the _conocardium_, a curious form that looks like a
_cardium_ cut through the middle, with a long slender tube added to
the dismembered side (Fig. 25). The _aviculopecten_, a shell allied to
our common scallop, and sometimes showing still its colour-bands (Fig.
25), and the _cardinia_ or _anthracosia_, a small bivalve that abounds
in the shales and ironstones of our coal-fields, along with nautili,
producti, and conulariæ at Coalbrook Dale, and with a thin leaf-like
lingula at Borrowstounness.
[Illustration: Fig. 25.--Carboniferous Lamellibranchs.
1. Aviculopecten sublobatus (showing colour-bands). 2. Conocardium
aliformis.]
The _Gastropods_ of the carboniferous rocks in the British Islands
embrace from twenty-five to thirty genera, with upwards of 200
species. Here, too, we can detect some forms that have not yet passed
away. The _trochus_, so universally diffused over the globe at the
present day, also lived in the palæozoic seas. Its companions, the
_natica_, the _turritella_, and the _turbo_, likewise flourished in
these ancient waters. Among the genera now extinct we may notice the
_euomphalus_, with its whorls coiled in a flat discoidal form; and the
_bellerophon_, with its simple coiled shell, resembling in general form
the nautilus. The gastropods are numerously represented in our gardens
and woods, by the various species of the snails, animals that have a
most extensive distribution over the world, and number probably not
much under two thousand species.
[Illustration: Fig. 26.--Carboniferous Gastropods.
1. Euomphalus peatangulatus. 2. Pleurotomaria carinata (showing
colour-bands).]
For a long time it was matter of surprise that no such land shells
had ever been detected in the carboniferous rocks. Trees and forests
had been turned up by the hundred, but never a trace was found of
any air-breathing creature. From this fact, and from the enormous
amount of vegetable matter preserved, it was once hastily inferred
that the atmosphere of that ancient period must have been uncongenial
to air-breathers; that, in short, it was a dense heated medium
of noxious carbonic-acid gas, wrapt round the earth like a vast
mephitic exhalation, favourable in the highest degree to the growth
of vegetation, yet deadly as the air of Avernus to all terrestrial
animals. But this notion, like most other bold deductions from merely
negative evidence, has had to be abandoned, for traces of air-breathers
have at last been found. Among these, not the least interesting is
the shell of a _pupa_, a sort of land-snail, which Sir Charles Lyell
detected, along with the bones of a small reptile, embedded in the
heart of an upright sigillaria stem in the carboniferous rocks of Nova
Scotia. Small as was the organism, the evidence furnished by it proved
scarcely less valuable than if it had been a large mammal that might
have afforded material for weeks of study. The similarity of the shell
to existing forms, showed that the ancient carboniferous forests had
at least one race of air-breathing creatures among their foliage, and
that the atmosphere of the period could have differed in no material
point from that of the present day, for as the snails breathe by lungs,
and require, consequently, a continual supply of oxygen to support
respiration, they could not have existed in an atmosphere charged with
carbonic acid.
[Illustration: Fig 27.--Carboniferous Cephalopods.
1. Nautilus Koninckii. 2. Goniatites crenistria. 3. Orthoceras laterale
(fragment).]
The _Cephalopods_, or highest class of mollusca, are represented among
the British carboniferous strata by seven genera. Of these the most
characteristic is the _orthoceras_, so named from its shell being
like a long straight horn. When the animal was young it inhabited a
single-chambered shell like that of many of the gastropods, but as
it increased in size and prolonged its shell in a straight line, it
withdrew from the first occupied chamber. This was partitioned off by a
thin wall called a _septum_, through the centre of which a tube ran to
the narrow end of the shell (Fig. 27). As the creature grew, chamber
after chamber was in this way formed, each of them quite air-tight, and
traversed by the central tube. Suppose a graduated series of diminutive
watch-glasses to be pierced by a long tapering glass-tube in such a
way that they should have their convex faces towards the narrow end of
the tube, and be arranged at short intervals, the smallest one placed
near the point of the tube, and the largest a little below the wider
end. Suppose, further, that this piece of mechanism were placed within
another tube tapering to an obtuse point, and that the edges of the
watch-glasses fitted tightly to the inner surface of this larger tube.
Such would be a rough model of the structure of the orthoceras.
The inner tube that traverses the centre of the chambers from end
to end of the shell is called the _syphon_, but its uses are very
problematical. At one time naturalists inclined to regard it as
intended to be filled with fluid, which, by expanding the membrane
of the tube, would compress the air in the chambers, and thus,
increasing the specific gravity of the animal, enable it to sink to
the bottom. In this way, by emptying or filling the syphonal tube,
the orthoceras might have risen rapidly to the surface of the deep,
or sunk as swiftly to the bottom. But this view, so pretty that one
wishes it were confirmed, must be regarded as at least doubtful. The
orthoceras more probably owed its power of progression to the action of
a funnel connected with the breathing apparatus, whereby jets of water
were squirted out that drove the shell rapidly along. The use of the
air-tight chambers was, perhaps, to give buoyancy to the shell so as to
make it nearly of the same specific gravity as water. Such a provision
must have been amply needed, for Professor Owen mentions an orthoceras
from Dumfries-shire that measured six feet in length, and similar
gigantic specimens have been found in America. Unless the chambers in
these shells had been air-tight, the animals that inhabited them would
have been held down about as firmly to one spot as if they had been
tied to a sheet-anchor. No mollusc could have possessed much locomotion
with so ponderous a tail, six feet or more in length, to drag after it.
But this inconvenience was obviated by the simple plan of having the
chambers close, and filled with nitrogen or other gas evolved by the
chemistry of the inmate. The shell, in this way, acquired no little
buoyancy, and probably stood up like a church spire, the animal keeping
close to the bottom to lie in wait for any hapless mollusc or trilobite
that might chance to come in its way.
The _nautilus_ (Fig. 27), which still lives in our seas, occurred
likewise in those of the Carboniferous period. It was a coiled shell;
in truth, just an orthoceras rolled up in one plane like a coil of
watch-spring. An allied form, called the _goniatite_ (Fig. 27), had
the margins of its septa of a zig-zag form, like the angles of the
wall round a fortified town. When the thin outer coating of the
shell is removed, the ends of these partition-walls are seen to form
strongly-marked angulated sutures or joints, where they come in contact
with the shell. Hence the name of the genus--_angled_ shell.
All these animals were predaceous. They did not confine themselves to
the lower forms of life, polyps and medusæ, nor even to the humbler
tribes of their own sub-kingdom, but hesitated not to wage war with
creatures greatly higher in the scale of creation than themselves, such
as the smaller fishes. They swarmed in the palæozoic seas, and well
merited the title of scavengers of the deep, that has been bestowed
on the sharks of our own day. They seem to have performed a function
now divided partly among the fishes and partly among the higher
gastropodous molluscs. And accordingly we find that as these latter
tribes increased, the orthoceratites, and goniatites, and ammonites
waned. At the present day, of all the palæozoic cephalopods there
remains but one--the nautilus[40]; a and so rare is it, that up to
the year 1832, all sorts of fanciful notions existed as to its nature
and functions. In fact, the nautilus was a sort of myth which any
naturalist could dress up as he chose, much as the old poets used to
picture the ship Argo. A specimen was at length procured and intrusted
to the examination of Professor Owen, by whom its anatomy was studied,
and afterwards philosophically described in an elaborate monograph.
Then, for the first time, did geologists obtain a true notion of the
nature of those siphonated shells, which lie grouped by hundreds in the
palæozoic and secondary formations. Yet we still want an account of the
habits of the nautilus. The older naturalists alleged that it could
at pleasure rise to the surface or sink into the depths of the ocean;
that it could spread out its fleshy arms and float across the waves or
draw them in, capsize the little vessel, and so return to a creeping
posture among the sea-weed at the bottom. These statements may to some
extent be true, for the chambers of the nautilus shell must impart
great buoyancy to it. But in the meantime the story of the sailing
propensities of the animal is derived from a sort of mythic age, and
must be viewed with some little suspicion. Until further observations
are made, we shall neither fully understand the economy of the nautilus
nor the habits of the cephalopods of the palæozoic seas. But the day
is probably not far distant when such doubts will be set at rest, and
we shall know whether the nautili and orthoceratites swam in argosies
over the surface of the ocean, or, keeping ever at the bottom, left the
waves to roll far above them, unvaried save perchance by some floating
sea-weed or drifted tree.
[Footnote 40: And perhaps even that is doubtful, for it is not unlikely
that after all, the palæozoic nautili may belong in reality to another
genus. Twenty years hence will probably see no little change on our
present identifications.]
CHAPTER VII.
Classification of the naturalist not always correspondent with
the order of nature--Incongruous grouping of animals
in the invertebrate division--Rudimentary skeleton
of the cephalopods--Introduction of the vertebrate
type into creation--Ichthyolites of the carboniferous
rocks--Their state of keeping--Classification of fossil
fishes--Placoids--Ichthyodorulites--Ganoids--Their structure
exemplified in the megalichthys and holoptychius--Cranium of
megalichthys--Its armature of scales--Microscopic structure of
a scale--Skeleton of megalichthys--History of the discovery
of the holoptychius--Confounded with megalichthys--External
ornament of holoptychius--Its jaws and teeth--Microscopic
structure of the teeth--Paucity of terrestrial fauna in coal
measures--Insect remains--Relics of reptiles--Concluding
summary of the characters of the carboniferous fauna--Results.
The organic remains hitherto described belong to that large division
of the animal kingdom instituted by Lamarck, to comprehend all those
whose internal structure is supported by no vertebral column, and
which are hence termed invertebrate. They are for the most part
protected by a hard outer covering, or exo-skeleton, which assumes
many different modifications. We have seen it in the calcareous
cells of the little net-like fenestella, in the geometric cup of
the stone-lily, in the double case of the cypris, and in the shells
of the mollusca. But the order of nature does not always exactly
correspond with the classification of the naturalist. His system
must necessarily be precise, formal, and defined. One tribe ends off
abruptly, and is immediately succeeded by another, with different
functions and structure, and dignified with a separate name. But in
the order of creation, such abrupt demarcations are few, for if they
exist in the present economy, they can not unfrequently be filled up
from the existences of the past. There is usually a shading off of
one class into another, like the blending of the tints of sunset,
and it often baffles all the skill of the profoundest anatomist, by
drawing a distinct line, to pronounce where the one division actually
ends and the other begins. Any name, therefore, which is intended to
embrace a large section of the animal kingdom, must ever be more or
less arbitrary. It will extend too far in one direction, and embrace
organisms which might be classed in a different section. It will
probably not extend far enough in another, and thus leave beyond its
pale animals possessing strong affinities to the majority of those
included under it. More especially is this true of every system of
classification that proceeds upon the modifications of a single
feature, or upon mere negative resemblances. Suppose, for instance,
that it were proposed by some highly systematic individual to divide
the inhabitants of our country into two great classes--the bearded
and the beardless. In the latter category he would arrange all the
more quiet and orderly portion of the community, with perchance a
tolerable intermixture of rogues. The bearded group would present a
most motley array--from the fierce-visaged heroes of the Crimea to the
peaceable stone-mason or begrimed pitman--all brought into one list,
and yet agreeing in no single feature save that of being like Bully
Bottom the weaver, "marvellous hairy about the face." But Lamarck's
invertebrate division of the animal kingdom presents a grouping of yet
more diverse characteristics, as cannot fail to be confessed when we
recollect that it embraces among its members the microscopic monad,
the coral polyp, the lobster, the butterfly, the limpet, the nautilus,
and the cuttle-fish. Cuvier's three-fold grouping of the division
into _mollusca_, _articulata_, and _radiata_, has now supplanted the
old name, though the latter is still retained as a sort of convenient
designation for all the animals below the vertebrate type.
The most highly developed of the recent cephalopods exhibit a true
internal skeleton, in the form of a strong oblong bone, on which the
body is hung. In this respect they occupy a sort of intermediate place
between the lower molluscs on the one hand, and the lower fishes on
the other. Theirs is not a vertebral column, but rather, as it were,
a foreshadowing of it; not, however, as a link in some process of
self-development from mollusc to fish, for these higher cephalopods do
not appear to have been created until fishes and reptiles had lived
for ages. The vertebrate type has been traced well-nigh as far back
into the past as we have yet been able to penetrate. Once introduced,
it has never ceased to exist, but in the successive geological ages
has been ever receiving newer and higher modifications, reaching its
perfection at length in man. The vertebrate form of structure fulfils
the highest adaptations of which terrestrial beings seem capable. We
can hardly conceive of corporeal existence reaching a more elevated
stage of development, save in thereby becoming less material, and
receiving an impartation of some higher element. The vertebrate
animals display not merely the most complexly organized structures,
but manifest in their habits the workings of the higher instincts
and affections. Among the invertebrate tribes the propagation of the
species is, in the vast majority of cases, a mere mechanical function,
like that of feeding or respiration, and the eggs once deposited, the
parent has no further care of her young. But among the vertebrated
animals, on the other hand, the perpetuation of the race forms the
central pillar round which the natural affections are entwined. It
parcels out every species into pairs, in each of which the mates are
bound together by the strongest ties of attachment. It gives birth,
too, to that noble instinct which leads the mother to expose her own
life rather than suffer harm to come to her offspring. It produces, at
least in man, that reciprocal attachment of offspring to parent, from
which springs no small part of all that is holiest and best in this
world. These attributes, to a greater or less extent, belong to all
the vertebrate animals, from the fish up to man. In looking over the
relics of animal life in the earlier geological formations, we are apt,
as we gaze on the massive jaws and teeth, the strong bony armour, and
the sharp, barbed spines, to think only of a time of war and carnage,
when the larger forms preyed upon the smaller, or ruthlessly sought
to exterminate each other. Yet should we not remember, that with all
these weapons and instincts of self-preservation there were linked
attributes of a nobler kind; that the earliest vertebrate remains point
to the introduction--though perhaps in but a rudimentary form--of
self-sacrificing love into our planet? The march of creation from
the first dawn of life has ever been an onward one, as regards the
development not only of organic structure but of the social relations;
and if it be true that physical organization finds its archetype in
man, it is assuredly no less so that in him too we meet with the
highest manifestation of those instincts which, by linking individual
to individual, have ever marked out the vertebrate tribes of animals
from the more machine-like characteristics of the invertebrate.
We pass now to the vertebrate animals, and shall look for a little into
the general grade and organization of the fishes that characterized the
carboniferous rivers and seas.
A collection of the ichthyolites of the carboniferous rocks presents
almost every variety in the mode of preservation. The smaller species
are frequently found entire, and show their shining scales still
regularly imbricated as when the creatures were alive. The larger forms
seldom occur in other than a very fragmentary condition. The limestones
yield dark-brown or black, oblong, leech-like teeth, which are found
on examination to be those of an ancient family of sharks. The shales
are often sprinkled over with glittering scales and enamelled bones.
Some of the coals and ironstones yield in abundance long sculptured
spines, huge jaws bristling with sharp conical teeth, and detached
tusks, sometimes five or six inches long. In short, the naturalist who
would decipher the ichthyology of the Coal formation, finds before
him, in the rocks, not a suite of correctly arranged, and carefully
preserved skeletons, but a set of disjointed, unconnected bones; here
a tooth, there a scale, now a jaw, now a dermal plate, all mingled at
random. And yet, though the evidence lie in this fragmentary state, our
knowledge of these ancient fishes is far from being correspondingly
meagre. To such precision has the science of comparative anatomy
arrived, that a mere scale or tooth is often enough to indicate the
nature and functions of the individual to which it belonged, and to
establish the existence in former times of a particular class or order
of animals. Thus the smooth rounded teeth of the mountain limestone are
found to present both externally and internally a close resemblance to
the hinder flat teeth of the sole living cestraciont (_C. Philippi_);
and we hence learn that a family of sharks, now all but extinct,
abounded in the palæozoic seas. The occurrence of a set of dark,
rounded little objects, which by the unpractised eye would be apt to be
mistaken for pebbles, is in this way sufficient at once to augment our
knowledge of the various animals of the Carboniferous period, and to
establish an important fact in the history of creation.
Of the four great Orders into which Agassiz[41] subdivided the class
_Pisces_, the Placoids and Ganoids, agreeing on the whole with the
cartilaginous fishes of Cuvier, occur abundantly in the palæozoic
rocks, while the Cycloids and Ctenoids, answering to Cuvier's
osseous fishes, began in the Secondary formations, and are found in
all subsequent deposits. The two former reached their maximum in the
earlier geological ages, and have been gradually dwindling down ever
since, till now they are represented by comparatively few genera; the
two latter are emphatically modern orders; they have been constantly
increasing in numbers since their creation, and swarm in every sea at
the present day. The carboniferous ichthyolites belong, of course, only
to the two first-mentioned orders the placoids and ganoids.
[Footnote 41: The classification of Agassiz, which is certainly not
a little arbitrary and artificial, has been altered by Müller, a
distinguished German anatomist, whose arrangement has been modified
again by Professor Owen. See Owen's _Lectures on Comparative Anatomy_,
vol. ii. p. 47. There is far from anything like unanimity on the
subject. Every naturalist thinks himself at liberty to modify and
restrict the groupings of his predecessors or contemporaries, sometimes
without condescending to give synonyms or any clue by which one may
compare the rival classifications. The geological student cannot engage
in a more sickening task than that of ranging through these various
arrangements, and he must possess some self-command who can refrain
from throwing up the search in disgust. The best way of progressing is
to select some standard work and keep to it, until the characteristics
of the genera and families have been mastered, and as far as possible,
verified from actual observation. After such preliminary training, the
student will be more able to grope his way through the "chaos and dark
night" of synonyms and systems.]
The Placoid, or _Plagiostome_ fishes, are familiar to us all as
exemplified in the common thornback and skate of our markets. They are
covered with a tough skin, which either supports a set of tuberculed
plates as in the thornback, or a thick crop of small rounded bony
points or plates, as in the shagreen of the sharks. The head consists
of a single cartilaginous box. The spinal column is likewise formed
of cartilage, built up in the higher genera of partially ossified
vertebræ. The tail is heterocercal or unequally lobed, inasmuch as
the spinal column, instead of ending off abruptly as it does in the
herring, trout, and all our commoner fishes, passes on to the extreme
point of the upper half of the tail. This is a noticeable feature, for
it has been found to characterize all the fishes that lived in the
earlier geological periods. The fins are often strengthened by strong
spines of bone, which stand up in front of them and serve the double
purpose of organs of progression and weapons of defence. The teeth vary
a good deal in form. In the larger number of existing placoids they
are of a sharp cutting shape, often with saw-like edges. Among the
sharks they run along the jaws in numerous rows, of which, however,
only the outer one is used, those behind lying in reserve to fill up
the successive gaps in the front rank. The teeth do not sink into the
jaw, as in the ganoids, but are merely bound together by the tough
integument which forms the lips. Another form of tooth, abundant among
the ancient placoids, and visible on some of those at the present
day, shows a smooth rounded surface, the teeth being closely grouped
together into a sort of tessellated pavement which, in the recent
species, runs round the inner part of the jaws, while a row of conical
teeth guards the entrance of the mouth.
[Illustration: Fig. 28.--Ctenacanthus hybodoides. (Edgerton.)]
The animals which possess these characteristics include the various
tribes of the sharks and rays, and form the highest group of fishes.
They are all active and predaceous, frequenting every part of the ocean
where their prey is to be found. The formidable spines and hideous
"chasm of teeth" belonging to the bulkier forms, render them more than
a match for any other denizens of the deep, and thus they reign in
undisputed supremacy--the scourge of their congeners, and a terror to
man.
The seas of the Carboniferous era abounded with similar predaceous
fishes, some of which must have been of enormous size. An entire
specimen has never been obtained; nor, from the destructible nature
of the animal framework, can we expect to meet with one. But the hard
bony parts of the animals, those capable in short of preservation
in mineral accumulations, are of common occurrence in the mountain
limestone beds and even among the coal seams. The dorsal spines or
_ichthyodorulites_, are especially conspicuous (Fig. 28). They stood
up along the creature's back like masts, the fin which was attached to
the hinder margin of each, representing the sail. The spine could be
raised or depressed at pleasure, its movements regulating those of the
fin, much as the raising or lowering of the mast in a boat influences
the lug-sail that is attached to it. The general form of these spines
was long, tapering, and more or less rounded. But they assumed many
varieties of surface ornament. Some species were ribbed longitudinally,
and had along their posterior concave side a set of little hooks
somewhat like the thorns of a rose. Others seem to have been quite
smooth, and of a flattened shape, with a thick-set row of sharp hooks
down both of the edges, like the spine on the tail of the sting-ray
of the Mediterranean. Such weapons have considerable resemblance to
the barbed spear-heads of savage tribes, and it is certain they were
intended to act in a similar way, as at once offensive and defensive
arms. The toothed spines of the sting-rays are still used in some
parts of the world to point the warrior's spear and arrow. Is there
not something suggestive in the fact that these stings, after having
accomplished their appointed purpose as weapons of war in the great
deep, should come to be employed over again in a like capacity on the
land; and that an instrument, which was designed by the Creator as a
means of protecting its possessor, should be turned by man into an
implement for gratifying his cupidity and satiating his revenge? Other
ichthyodorulites are elegantly ornamented by long rows of tuberculed
lines arranged in a zig-zag fashion, or in straight rows tapering from
base to point. In all there was a blunt unornamented base, which sank
into the back and served as a point of attachment for the muscles
employed in raising or depressing the spine. In some specimens the
outer point appears rounded and worn, the characteristic ornament being
effaced for some distance--a circumstance which probably indicates that
these fishes frequented the more rocky parts of the sea.[42]
[Footnote 42: See Egerton, _Quart. Jour. Geol. Soc._, vol. ix. p. 281.]
The placoid teeth of the carboniferous rocks show the usual forms
of the order. Some of them are sharp and pointed, as those of the
hybodonts; others have a smooth, rounded, or plate-like form, as in the
cestracionts. The latter often show a dark brilliant surface, and might
be readily enough mistaken for well-worn pebbles. In the oblong rounded
teeth of _psammodus_ the surface is densely covered with minute points
like grains of sand, whence the name of the genus. These teeth, when
sliced and viewed under the microscope by transmitted light, exhibit a
complex reticulated internal structure.
Agassiz' second great Order of fishes is named Ganoid, from a Greek
word signifying brightness, in allusion to the brightly enamelled
surface of their dermal covering. They differ from the placoids in
having their outer surface cased in a strong armature of bone, which
is disposed either in the form of large overlapping plates, as among
the strange tortoise-like fishes of the Old Red Sandstone, or as thick
scales, which are either placed at intervals, as along the back and
sides of the sturgeon, or closely imbricated, as in the stony-gar
(_lepidosteus_) of the American rivers. This strong, massive skeleton
constitutes in many genera the sole support of the animal framework,
the inner skeleton being of a gristly cartilaginous kind, like that
of the skate. On this account traces of the vertebral column are by
no means abundant among the older formations. But as the ganoids form
a sort of intermediate link between the placoid or gristly fishes
on the one hand, and the bony fishes on the other, they are found
to present in their different genera examples of both these kinds
of structure. Thus, the skeleton of the sturgeon consists of a firm
cartilage, out of which the vertebræ are moulded, so that this fish
was at one time ranked with the sharks in the cartilaginous tribe of
Cuvier. The skeletons of some of the older ganoids (as _holoptychius_),
on the other hand, manifest such a decidedly osseous structure, with
sometimes so much of a reptilian cast, that the bones were at first
referred to some huge extinct saurians. The head of the ganoid fishes
is encased in a set of large massive plates of bone, and the jaws
are furnished with several rows of small sharp teeth, intermingled
with a less numerous but larger-sized and more formidable kind. The
interior of the mouth likewise displayed in many ancient genera groups
of palatal teeth, so that the dental apparatus of these animals
must have been very complex and complete. The tail in all the older
ganoids was heterocercal, like that of the sharks, the lobes being
not unfrequently densely covered with minute overlapping scales of
bone--a peculiarity which also extended to the fins. But the fins
were sometimes strengthened in another way by having the foremost ray
greatly thickened and enlarged, so as to form a stiff spine like the
ichthyodorulites of the placoids. The whole of the external surface
of these ganoidal fishes glittered with enamel, and was usually
sculptured in the most graceful patterns or ornamented with fine
lines and punctures so minute as to be almost invisible to the naked
eye. Every plate, scale, fin-ray, nay, the very lips exhibited the
characteristic enamel mottled over with the style of ornament peculiar
to the species. And when we think we have exhausted the contemplation
of these beauties, it needs but a glance through an ordinary microscope
to assure us that the unassisted eye catches only a superficial glimpse
of them. The more highly we magnify any portion of these old-world
mummies, the more exquisite does its structure appear.
In the carboniferous rocks of Great Britain, upwards of forty species
of ganoids have been detected. They have a wide range in size, the
smallest measuring scarce two or three inches, while the largest, to
judge at least from the bones which they have left behind, must have
reached a length of twenty, or perhaps even thirty feet The lesser
genera (Fig. 29) were characterized by small, angular, glossy scales,
usually ornamented either with a very minute punctulation, or with
fine hair-like lines which sometimes exhibited the most complicated
patterns. The scales were likewise occasionally serrated along the
exposed edges--a style of ornament which gives no little richness to
the aspect of the dermal covering. The fins, closely imbricated with
small angular scales of bone, sometimes displayed a striated ray in
front, but this neither possessed the strength nor the formidable
aspect of the corresponding spine among the placoids. The head was
encased in a set of bony plates fitting tightly into each other, and
ornamented with various patterns according to the species. The teeth
were very small and fine, resembling the bristles of a brush, but in
at least some species intermingled with teeth of a larger size. The
minute style of dentition in these smaller fishes has been thought to
indicate their habit of keeping to the bottom of the water and feeding
on the soft decaying substances lying there. Nowhere have I seen the
small rhomboidal scales of the _palæoniscus_ so abundant as among dark
shales charged with cypris cases and fragments of terrestrial plants,
and on such occasions the idea has often occurred that these graceful
little fishes, like the _amia_ of the American rivers, may have fed on
the cyprides that swarmed along the bottom of the estuary.
[Illustration: Fig. 29.--Amblypterus macropterus (a Carboniferous
ganoid).]
Scattered over the fresh-water limestones, ironstones, and shales, or
crowded together along the upper surface of some of the coal-seams,
there occur the remains of two very remarkable ganoidal fishes. They
deserve our attention for their great size, their complex organization,
and the important place in the scale of animal life which they occupied
during a former period. One of them has been called _megalichthys_
or _great fish_--an unhappy name, since the animal did not reach the
dimensions attained by not a few of the other ganoids, and was even
surpassed by at least one of its contemporary congeners. The other
is known as the _holoptychius_ or _wrinkled scale_. A more detailed
examination of these two animals will perhaps best enable us to
understand the character of the ganoid fishes that lived in the waters
of the Carboniferous period.
The megalichthys had an average length of about three feet. Like
the other members of the ganoid order it had a glittering exterior,
every scale and plate being formed of strong bone, and coated with a
bright layer of enamel. Wherever this polished surface extends, it is
found to be ornamented with a minute punctulation, the pores of which
lie thickly together like the finer dots of a stippled engraving.
The cranial plates are further varied by a scattered and irregular
series of larger punctures that look as if they had been formed by the
insertion of a pin-point into a soft yielding surface. The examination
of the head of the megalichthys as depicted in Fig. 30, will convey an
adequate conception of the structure of a ganoidal cranium.
[Illustration: Fig. 30.--Head of Megalichthys Hibberti, one-sixth of
natural size (Agass. _Poiss. Foss._ Tab. 63).
A Upper side. B. Under side. C Profile.]
The snout is formed of an elegantly curved bone (_c_) fringed along
its under edge with minute thick-set teeth. On either side it is
flanked by two triangular plates, which occupy the space between the
intermaxillary bone (_c_) and the upper jaws (_q q_). The eye orbits
seem to have been at the corners of the intermaxillary, circumscribed
by the sub-orbitals (_f g h_) and the ethmoids (_b_). The massive
intermaxillary bone had its posterior margin of an angular form, and
into the notch thus formed there was wedged the anterior end of a long
strip of plates, which expanded as they approached the occipital part
of the cranium, and terminated in three irregular plates that may
represent the place of the parietal and occipital bones. The space
between this belt and the upper jaws was occupied by three large plates
(_i k l_) which in other ganoids, as the _osteolepis_ of the Old Red
Sandstone, were united into a single pre-opercular bone of considerable
size. The operculum or gill cover (_m_) was relatively large, and
had an elegantly curved anterior margin. The upper jaws (_q_) were
comparatively small, and had a fringe of small conical teeth. The under
jaws (_r r_) reached to nearly double the length of the upper, and were
similarly set round with teeth. The teeth of the megalichthys, like
those of the living lepidosteus, consisted of two kinds, of which the
one bristled thickly along the outer edge of the jaw as sharp minute
points, averaging about a line in length, while behind this outer row
lay a scattered series of much larger teeth that sometimes rose nearly
an inch above the jaw. The external surface of these more formidable
tusks is smooth, glittering, and minutely striated with fine lines from
base to point, while the root of each is farther marked by a circle of
short, deep, longitudinal furrows. The internal structure displays a
close ivory, which when viewed under a microscope is seen to be made up
of fine tubes radiating from the outer surface to the hollow central
cavity. Some of the bones in the interior of the mouth seem to have
been also furnished with an apparatus of teeth. The under surface of
the cranium between the arch of the under jaws consists of two oblong
central plates (_t_) surrounded by a row of sixteen irregular ones,
eight on each side, and terminated in front by a large lozenge-shaped
scale (_u_) which fits into their angle of junction on the one side,
and into the symphysis of the jaws on the other. In the osteolepis
there were likewise two large plates terminating in a similar
lozenge-shaped one, but without the flanking rows. In the famous Old
Red holoptychius of Clashbennie, the under surface of the head had
but two plates, and in the still older and more gigantic asterolepis,
there was but one. It is the delightful task of the paleontologist
to compare and contrast these various pieces of mechanism, to mark
how what seems lacking in one comes to be supplied in another, and to
trace out the various modes in which, during the ages of the past,
Nature has wrought out the same leading plan, sounding, as it were, an
ever-changing series of modulations upon one key-note. In comparing
together the ganoids of the Old Red Sandstone and the Carboniferous
rocks, he finds that in the asterolepis--a fish belonging to the lower
part of the former formation--the pointed arch formed by the sweep of
the lower jaws is filled up by a single plate like some abbey-window
with its mullions knocked away, and built up with rude stone and lime.
Higher in the same group of rocks he meets with the cranium of the
holoptychius, where there is one straight central mullion running
in an unbroken line from the angle of the arch to its base. In the
osteolepis[43] he sees this mullion branching into two at its upper
end, so that the window consists of three divisions, as in the simplest
style of Gothic. Passing upwards into the Carboniferous system, he
encounters a still more ornate arrangement in the cranium of the
megalichthys. The central mullion with its two upper branches still
remains, but it is flanked by an additional one on each side, from
which there spring six cross bars that diverge obliquely with a slight
curve, so as to join the outer arch and subdivide the window into
nineteen compartments. So varied are the plans of the Divine Architect
in what to man may seem such a little matter as the piecing together of
a fish's skull.
[Footnote 43: Hugh Miller's _Footprints of the Creator_, p. 91.]
The body of the megalichthys was cased in an armature of as solid and
glittering bone as that which defended its head. Where the plates of
the cranium ended off they were succeeded by large rhomboidal scales
that crossed the body obliquely, and overlapped each other like the
metal plates in the antique scale-armour. Each scale consisted of
two parts, of which one had a rhomboid form and was covered over with
enamel, while the other ran round the two inner sides of the rhomb
as a broad unenamelled selvage deeply indented along its centre. It
was the enamelled portion alone that formed the outer surface, the
rough unpolished border being covered by the overlapping edges of
the adjoining scales. The scales had not a uniform thickness, but
were strongest at the covered part from which each thinned off to
the outer edges. In this way the thin edge of one scale pressed down
on the thick part of the subjacent one, and a covering of uniform
strength and smoothness was produced. Looking at a set of these scales
as they still occupy their original position on the creature's body,
it is scarcely more than a half of each which meets our eye; for the
unenamelled border occupied about a third of the entire surface, and
a fourth of the remainder was covered by the overlapping scales. The
effect of this arrangement must have been to combine great strength
with the most perfect flexibility. Notwithstanding the bulk of his
helmet and the weight of his scale-armour, we cannot conceive the
megalichthys to have been other than a lithe, active, predaceous fish,
dealing death and destruction among the herring-like shoals of little
palæonisci and amblypteri, though able to maintain perhaps but a
doubtful warfare with his more bulky contemporary, the holoptychius.
The internal structure of the scales of the megalichthys exhibits the
same provision for combining strength with the least possible amount
of material. Viewed in a transverse section under a magnifying power
of about eight diameters, they are seen to consist of three layers of
bone; each possessing a peculiar structure. The outermost is formed
of a tessellated pavement of minute round ocelli, having a fine brown
colour, and placed close together with considerable regularity. They
somewhat resemble little wheels, the axle being either a dark solid
nucleus or a small circular aperture, whence there radiates to the
outer rim a set of exceedingly minute fibres which were originally
hollow, and served as canals to carry on the growth of the scale. The
vacant space left where four wheels impinge on each other, forms one
of the pores that cover the enamelled surface of the scale. The whole
structure of this outer layer very closely resembles that presented by
the internal part of the base of the teeth, save that the confluent
lobes shown in the teeth become in the scale detached into separate and
independent circles. The central stratum of each scale is composed of
a loose open network of cancellated bone that passes into the layer on
either side, and resembles in its general texture the osseous vertebræ
of the same fish. The under layer, one end of which rested immediately
on the skin, approaches more to the firmness and solidity of the outer
one, but, in place of a tessellated, ivory-like pavement, it had a
close fibrous texture, with here and there a scattered cavity, and the
fibres were matted together so as to resemble the more solid structure
of the cranial bones. The effect of this triple arrangement must have
been to impart great strength and lightness to the external armature
of the fish; the middle spongy layer serving, by its porosity, at once
to deaden the effect of any blow aimed at the outside, and to give
buoyancy and lightness to what would otherwise have been a coat of
mail well-nigh, as ponderous as that of a feudal chief. One can hardly
conceive any implement of warfare in use among the lower animals of
strength enough to pierce this massive covering. But we shall find
as we go on that if the megalichthys had a strong defensive armour,
a bulkier neighbour had a still stronger offensive one, and that the
enamelled plates of the one fish were scarcely a match for the huge
pointed tusks of the other.
The megalichthys had an osseous skeleton, with vertebræ of a discoidal
form. These internal bones when viewed under the microscope are found
to display an open cancellated structure, resembling that of the
central layer in the scales. It thus appears that this ancient fish
was not merely defended by a hard external armour, but possessed an
equally solid framework of bone within.
Mingled with the scales and bones of the megalichthys, there are found
the remains of a still larger fish, to which the name of Holoptychius
has been given. Its external ornament differed entirely from that of
the animal last described. It possessed teeth sometimes six or seven
times larger, and jaws, plates, and bones of a form and dimensions
totally distinct. Strange as it may seem, however, these two fishes
have been constantly and systematically confounded from the time when
they were first discovered. Two or three years ago, there might be
seen in the British Museum several specimens of the holoptychius,
of which some bore the correct name, while the rest were labelled
"Megalichthys;" and a similar error prevailed in several of the other
museums.[44] The confusion can be traced very distinctly in the memoir
of Dr. Hibbert, who for the first time described the remains of these
fishes, and wrote according to information received from Agassiz.
[Footnote 44: The mistake was noticed in 1845 by Hugh Miller, who, in a
foot-note to his _First Impressions of England and its People_, p. 71,
well defines the distinctions between the two ichthyolites.]
In the autumn of 1832, the attention of the scientific public of
Edinburgh was directed to the extraordinary character of some fossil
remains obtained from the lime-quarries of Burdiehouse, a village about
four miles to the south of the town. Dr. Hibbert visited the locality,
and soon saw enough to excite his lively interest in its thorough
investigation. The Royal Society of Edinburgh warmly supported his
exertions, and by their means a large suite of specimens was eventually
obtained, which the Doctor from time to time described as they were
successively received. At the meeting[45] of the British Association in
Edinburgh, in 1834, the specimens were exhibited before the Geological
Section, and a memoir upon them read by their successful discoverer.
On the conclusion of the paper, a lively discussion ensued upon the
nature of the animal to which the scales and teeth had belonged.
Dr. Hibbert argued, from the deeply-furrowed teeth, and the strong,
massive cranial plates, that the animal must have been a reptile, and
supported his assertion by no small amount of anatomical skill. In the
midst of the discussion, a message was sent to the great ichthyologist
of Neufchatel, who happened to be at that time busily engaged in the
Zoological Section. Passing over the fossils as they lay grouped
upon the table, with that quick perception for which he is so justly
celebrated, Agassiz at once decided that the bones must have been
those of some large and hitherto undescribed _fish_. Such a decision
from such an authority produced of course no little sensation, and the
naturalist was told with some surprise that the remains had just been
elaborately described as those of extinct reptiles. "Reptiles!" thought
Agassiz, and again his quick eye darted over the table; but the fossils
would yield no other answer than what they had already given. Despite
their seeming reptilian character, they were undoubtedly ichthyic,
though belonging to an animal up to that time unknown. In the completed
memoir which Dr. Hibbert subsequently submitted to the Royal Society,
his mistake was freely acknowledged, and the remains there flourish as
those of a true fish. But with this amendment a grave error of another
kind was committed, though in this the Doctor seems to have been
supported by the authority of Agassiz himself. The large bones, scales,
and teeth of the Burdiehouse limestone, were all indiscriminately
thrown into one genus, to which Agassiz gave the name of Megalichthys;
and in the memoir we find the different kinds of scales and teeth
described and figured without the slightest intimation or suspicion
that they might possibly have belonged to different animals. The
novelty of the discoveries soon attracted general attention to
Dr. Hibbert's paper. It was quoted or referred to in almost every
scientific work treating of general geology, while in some instances
(as in Dr. Buckland's _Bridgewater Treatise_) the erroneously-named
bones were re-engraved. A tooth from the Fife coal-field, drawn
for one of the woodcuts in a popular elementary manual, was also
named megalichthys; an error perpetuated through every edition till
the last, where the tooth has been restored to its true owner--the
holoptychius. In truth, no two organisms have ever been so maltreated;
and if the reader will kindly bear with me a little further, it will
not be difficult to show him that the holoptychius had peculiarities
of its own quite as distinct as those that have come before us in the
megalichthys, and that each animal has a full and legitimate claim to a
separate and independent niche in the gallery of fossil fishes.
[Footnote 45: See Agassiz, _Poiss. Foss._, tom. ii. Part 2, p. 89 _et
seq._]
The word _holoptychius_ means, as I have said, "wrinkled or folded
all over,"--a name truly expressive of the peculiar style of ornament
displayed by every part of the exterior of the animal's body. The
head-plates, which are of great size, exhibit a fine corrugated
shagreen-like surface, roughened into knobs, and wavy lines of
confluent tubercules, that remind one disposed to be fanciful, of a
frosty December moon with its isolated peaks, and confluent mountain
chains. The scales are of a rounded or oval form, and vary from less
than half an inch to fully four or even five inches in diameter.
Their upper side consists of two parts, one of which with a crescent
shape lay beneath, the over-lapping scales, while the other passed
outwards to form a portion of the outer visible surface. The part
that was hidden by the overlapping scale was smooth, with a finely
striated surface. The exposed portion displayed the usual corrugated
sculpturing, many of the little tubercules having striated sides, and
showing, in consequence, no little resemblance to the star-like knobs
on the dermal covering of the Old Red Sandstone asterolepis. The inner
surface of the scales was concave, with a central prominent oblong
point surrounded by encircling scaly ridges, and forming what is called
the centre of ossification.[46]
[Footnote 46: The above descriptions of the scales and teeth of these
two fishes, are taken from specimens in my own collection. None of my
holoptychian scales show incontestable the proportion of the covered to
the exposed part. Judging from the aspect of one of them, the wrinkled
portion occupied perhaps about three-fifths of the entire scale,
the remaining part being covered by the overlapping edges of those
adjacent; for the characteristic corrugated surface was essentially
an external ornament, and ceased at the point where the external
bone passed into the interior. I may remark, that the upper side of
the scales is not very frequently seen in the Burdiehouse limestone,
the rough surface usually adhering to the rock, and leaving only the
smooth inner side exposed. Out of seven specimens from that locality,
only one shows the upper side, and that by no means in a perfect state
of keeping. The structure alike of scales and bones can be seen to
much greater advantage in the shales, ironstones, and coals of the
coal-fields, where, owing to the soft nature of their matrix, the
fossils can be readily cleared and exposed.]
But perhaps the most remarkable and characteristic parts of the
carboniferous holoptychius were its jaws and teeth. As we might
readily conjecture from the great size and strength of the scales and
cranial plates of this fish, its dentition was of a correspondingly
massive type. The under jaw, with the usual corrugated ornament,
frequently exceeded a foot in length, and displayed along its upper
edge a thick-set group of teeth. Of these there were two kinds one
of a smaller size and more blunted form, with short indented furrows
at their base; the other of a greatly more formidable size, grouped
at intervals among the smaller ones. The front end of each under jaw
bore one of these long conical tusks, serving as it were to guard the
entrance of the mouth. Each of the larger teeth had a base strongly
marked with longitudinal furrows, and sank deep into the jaw, with
the bone of which it sometimes anchylosed.[47] The part of the tooth
above this socket had an oval form, so flattened as to present two
cutting edges, one facing the front, the other the back of the mouth,
and meeting at the upper end of the tooth which was sharp and pointed.
Such large conical tusks may frequently be obtained, having a length
of two or three inches, while occasionally they range as high as six
or seven, the smaller teeth seldom reaching so much as an inch. It
is difficult to see how, with such a formidable dentition, the jaws
could readily close. In some specimens I have seen deep hollows beside
the bases of the teeth, which may possibly have received those of the
opposite jaw, but the gigantic tusks at the entrance of the mouth seem
to have stood high over the jaw, passing outside like those of the
wild-boar. If this be correct, the jaw of the holoptychius would unite
the mechanism of both the alligator and the crocodile--its recipient
hollows being analogous to the tooth-pits in the former tribe, and its
protruded teeth to the similarly exposed teeth of the latter.
[Footnote 47: I have seen detached teeth, wherein the length of
the root, or part imbedded in the jaw, tripled that of the exposed
part, sinking four or five inches into the bone without any trace of
anchylosis. Whether these huge tusks belonged to the upper or under
maxillary, I do not pretend to say, though no specimen of the under
jaw, which has ever come under my notice, would accommodate half of
such a deep-sunk base.]
[Illustration: Fig. 31.--Jaw of Holoptychius (_Rhizodus_.--Owen) from
Gilmerton, one-fourth nat. size; the large teeth along the middle part
of the jaw are here wanting.]
When we bring the microscope to bear upon the elucidation of the
structure of these ancient teeth, it seems as if our labour had but
just begun; and that so far from having by an external scrutiny
exhausted all that they have to show us, our knowledge of them can be
but scanty and superficial until we have studied them carefully under
a magnifying power. Microscopic sections of such organic remains are
prepared in the same way as those of the fossil woods already noticed;
and a more interesting or beautiful series of objects cannot be
conceived than a set of slices of these fossil-teeth.
Viewed, then, in longitudinal section from base to point, the part
above the fluted root of one of the large teeth of the holoptychius is
seen to consist of minute hair-like fibres of extreme tenuity, which
proceed in straight lines from the outer surface to the interior. At
right angles to these, and parallel with the outer edges, there is
a set of dark widely-placed lines conforming to the outline of the
tooth, like so many long sugar-loaf shaped caps, placed within each
other. When this part is cut across, and viewed in transverse section,
the tooth is observed to be of a flattened oval form, with the same
fine fibres or tubes radiating from the centre, and traversed by the
same dark bands which now assume the form of concentric rings. The
appearance thus presented reminds one at once of a cross section
of some dicotyledonous tree, the dark bands resembling the annual
layers of growth, and like these resulting from a similar thickening
of the internal tissue. The upper part of the tooth is solid and the
concentric rings few; the middle exhibits an increase of the rings, and
possesses, moreover, a hollow centre or pulp-cavity,[48] with the usual
diverging fibres. Here the oval form is well shown, and the encircling
rings are considerably flattened at the ends of the long axis.
[Footnote 48: This hollow centre may be seen occasionally filled up by
a sharp conical tooth like the _phragmocone_ of a belemnite.]
The lower portion of the tooth exhibits a much more complicated
texture. Externally it is marked by deep longitudinal furrows, that
run down the enamelled sides and sink into the jaw. When cut across
at this ribbed part, the tooth is found to present the most complex
and graceful internal structure. The prominent ridges between the
furrows are seen to be produced by crumpled folds of the substance of
the tooth, which roll inwards towards the centre, coalescing with each
other, and forming intricate groups of circling knots and folds. In
some places they seem all but separated from each other into little
circles, pierced with a central aperture, and recall the aspect of
the upper layer in the scale of megalichthys. Each of these loops and
folds presents a texture exactly similar to that of the upper part of
the tooth. The same minute hair-like tubes, darkened and thickened in
the long axis, radiate towards the centre; the same concentric bands
run from centre to circumference; so that the lower part of the tooth
seems, as it were, made up of a bundle of smaller teeth partially
melted into each other. Between these loops and folds circular meshes
frequently occur, and add to the complexity as well as the beauty of
the whole structure. One of these sections, with all its twisting
crumples, and folds, and knots, and coloured meshes, and encircled
rings, bears no small resemblance to an antique polished table that
has been cut out of the gnarled roots of a venerable oak. This complex
structure arose from the mode of growth of the tooth; each prominent
external ridge continually turning inwards down the furrow on either
side, and mingling in freakish knots with the folds that had gone
before.[49]
[Footnote 49: For an acquaintance with the remarkable teeth of this
ancient fish, more minute than it had been my good fortune to possess
before, I am indebted to a most interesting series of microscopical
preparations kindly lent me from his extensive collection by my friend
Mr. Alexander Bryson of Edinburgh.]
The internal bones of the holoptychius were of great size and strength,
as befitted such a bulky ganoid. Some of them had a singular style of
surface ornament, that somewhat resembled a frosted widow on a December
morning. Their internal structure was loose and cancellated; the endo-
being usually of a less compact texture than the exo-skeleton. Judging
from the size of such bones, the carboniferous holoptychius must have
been one of the bulkiest and most formidable denizens of the deep,
reaching sometimes to a length of twenty feet or even more. Such an
animal would have been, perhaps, quite a match for our hugest crocodile
or alligator, for it must have swum about with a litheness and agility
possessed by none of the saurian reptiles. Like that leviathan chosen
by the Almighty, in an age long subsequent, as an illustration of His
power and greatness, the holoptychius must have been king over all
the inhabitants of the sea, and the magnificent language of Job,
descriptive of the living animal, applies not less graphically to
the extinct one: "Who can open the doors of his face? his teeth are
terrible round about. His scales are his pride, shut up together as
with a close seal. One is so near to another, that no air can come
between them. He maketh the deep to boil like a pot: he maketh the sea
like a pot of ointment. He maketh a path to shine after him; one would
think the deep to be hoary."
Our survey has hitherto been directed to the denizens of carboniferous
lake, river, and sea, and we have found them to be alike important in
numbers and interesting in organization. It is otherwise, however,
when we turn in search of the denizens of the carboniferous lands.
The crowded trees and shrubs of the coal strata recalling as they do
old forest-covered swamps, might seem to indicate the probability of
a pretty numerous terrestrial fauna. Where are we to look for the
fossilized relics of land animals, if not in the remains of a submerged
land-surface? And yet, strange as it may seem, of the inhabitants of
the land during the Coal-measure period we know almost nothing. "We
have ransacked hundreds of soils replete with the fossil roots of
trees,--have dug out hundreds of erect trunks and stumps, which stood
in the position in which they grew,--have broken up myriads of cubic
feet of fuel, still retaining its vegetable structure,--and, after
all, we continue almost as much in the dark regarding the invertebrate
air-breathers of this epoch, as if the coal had been thrown down in
mid-ocean."[50]
[Footnote 50: Sir Charles Lyell's _Elements_, fifth edition, p. 406.]
The little land-shell already noticed as having been detected by Sir
Charles Lyell in the carboniferous rocks of Nova Scotia, seems to be
as yet the only air-breathing mollusc obtained from rocks of such
high antiquity. Insect remains have been detected in the English
coal-fields belonging to two or three species of beetles; while on the
Continent, wing-sheaths and other fragments of cockroaches, scorpions,
grasshoppers, locusts, crickets, &c., have been detected. But the most
remarkable traces of air-breathers consist in various indications of
the existence of reptiles during the Carboniferous era. Fragmentary
skeletons, with detached bones and plates, have been found in Bavaria
and America, together with long tracks of footprints, from which it
appears that during the time our coal-seams were forming, there swam
through the sluggish deltas, or crept amid the dank luxuriant foliage,
strange lizard-like forms, large enough to leave behind them on the
soft yielding mud or sand the impress of their double pair of toed
feet. But of these animals we have much to learn. Some of them have
bequeathed to us merely their dismembered broken bones; others have
left but the imprints of their toes. Yet even these remains, trifling
as they may seem, become of importance when we remember that they
demonstrate fishes not to have been the highest types of being during
the epoch of the Coal, and show that while the bulky holoptychius held
the supremacy of the waters, lizard-like forms of a less formidable
type seem, so far as we know, to have ruled it over the land.
In fine, then, no one can glance at a list of the carboniferous fauna
without perceiving either that the animated world of that ancient
epoch must have had a very different proportioning from what now
obtains, or that we have only a meagre and fragmentary record of it.
That the latter conclusion is the more philosophical will appear if we
reflect upon the many chances that exist against the entombment and
preservation of animal remains, especially of those peculiar to the
land. How very small a proportion of the remains of animals living in
our own country could be gathered from the surface-soil of any given
locality, and how very inadequate would be the meagre list of species
thus obtained, as representing the varied and extensive fauna of Great
Britain! In contrasting, then, the rich abundance of marine organisms
with the extreme paucity of terrestrial animals among the carboniferous
rocks, it would be too hasty to infer a corresponding disproportion
originally. It must be admitted that the rarity of air-breathers,
after such long-continued and extensive explorations among terrestrial
and lacustrine beds, presents a difficult problem, only (if at all)
to be cleared away by patient and persevering investigation. With
this preliminary caution, we may regard the carboniferous fauna as
peculiarly rich in marine species. The sea-bottoms swarmed with
stone-lilies, cup-corals, and net-like bryozoa, mingled with the
various tribes of molluscan life--the brachiopods with their long
ciliated arms; the bivalves and gastropods with their coloured shells
that recall some of the most familiar objects of our shores; and the
cephalopods with their groups of siphonated chambers, straight as in
the orthoceras, or gracefully coiled as in the goniatite. The seas
swarmed, too, with fishes belonging to the two great orders of ganoids
and placoids, the latter represented now by our sharks and rays,
though the exact type of the ancient genera is retained only by the
cestracion or Port-Jackson shark; the ganoids, with their strong armour
of bone, represented by but two genera, the lepidosteus of the American
rivers, and the polypterus of the Nile,--two fishes that seem but as
dwarfs when placed side by side with the gigantic holoptychius of the
coal-measures. The rivers and estuaries of the same period seem to have
been frequented by immense shoals of the smaller ganoidal fishes that
fed on decaying matter brought down from the land, and perhaps, too,
on the minute Crustacea that lay strewed by myriads along the bottom.
Into these busy scenes the bulkier monsters from the sea made frequent
migrations, perhaps in some cases ascending the rivers for leagues to
spawn, and returning again to their places at the mouth of the estuary
or in open sea. The rivers and lakes swarmed with small crustaceous
animals, and nourished, too, shells like those of our pearl-mussels.
The land--so luxuriantly clothed with vegetable forms--was hummed over
by beetles, chirupped over by grasshoppers and crickets, and crawled
over by four-footed reptiles, that united in their structure the lizard
and the frog. But of the general grade and proportions of its denizens
we still remain in ignorance. From all that yet appears, the scenery
of these forests must have been dark, silent, and gloomy, buried in a
solitude that was startled by no tiger's roar, no cattle's low, and
neither cheered with the melody of birds nor gladdened by the presence
of man.
We have lingered, perhaps, too long over the remains of these old
carboniferous animals. But the delay may be not without its use if, by
thus bringing before us some of the more marked points in the structure
of creatures that for ages peopled our planet, it broaden our view of
creation; and by lifting the curtain from off a dim, distant period of
our world's long history, it show, amid all diversities of arrangement,
and all varieties of form, still the same grand principles of design,
and the same modes of working as those which we can see and compare
among the living forms around us. It is something to be assured that
the race of man has been preceded by many other races, lower indeed
in the scale of being, but manifesting, throughout the long centuries
of their existence, ideas of mechanism and contrivance still familiar
to us, and serving in this way to link the human era with those that
have gone before, as parts of one grand scheme carried on by one great
Creator.
CHAPTER VIII.
Sand and gravel of the boulder--What they suggested--Their
consideration leads us among the more mechanical operations
of Nature--An endless succession of mutations in the economy
of the universe--Exhibited in plants--In animals--In
the action of winds and oceanic currents--Beautifully
shown by the ceaseless passage of water from land to
sea, and sea to land--This interchange not an isolated
phenomenon--How aided in its effects by a universal process
of decay going on wherever a land surface is exposed to
the air--Complex mode of Nature's operations--Interlacing
of different causes in the production of an apparently
single and simple effect--Decay of rocks--Chemical
changes--Underground and surface decomposition--Carbonated
springs--The Spar Cave--Action of rain-water--Decay of
granite--Scene in Skye--Trap-dykes--Weathered cliffs of
sandstone--Of conglomerate--Of shale--Of limestone--Caverns
of Raasay--Incident--Causes of this waste of calcareous
rocks--Tombstones.
From the blackened plants that darkened the upper layers of the
boulder, the transition was natural to the matrix in which they lay.
The whole rock consisted of a fine quartzy sand more or less distinctly
laminated, and showing in its lower parts well-rounded pebbles of
quartz, green grit, and felspathic trap. The contemplation of these
features suggested the existence of some old land with elevated ranges
of hills, and wide verdant valleys traversed by rivulets and rivers
which bore a ceaseless burden of mud, sand, and gravel, onwards to
the sea. The pebbles afforded some indication of the kind of rocks
that formed the hill-sides. Perhaps the higher grounds exhibited that
grey wrinkled appearance peculiar to the quartz districts of the
north-western Highlands, with here and there a bluff crag of felspathic
trap shooting up from among the fern-brakes of the valley, or cutting
across the channel of some mountain stream that tumbled over the pale
rock in a sheet of foam. And there may have been among these uplands
smooth undulating districts, dotted over with dark araucarian pines,
and densely clothed with a brushwood of rolling fern, but which showed
in all their ravines the green grit that formed the framework of the
country,--its beds twisted and contorted, jointed and cleaved, like
the grits and slates along the banks of many a stream, beloved by
the angler, in the classic ground of the Ettrick and the Yarrow. But
whatever may have been the special features of its scenery, there can
be no doubt of the land's existence. The carbonized plants stand up to
tell us of its strange and luxuriant vegetation. We have listened to
their story, and suffered them to lead us away into forest, and lake,
and sea, to look on the various forms of life, vegetable and animal,
which abounded in that far-distant age. We return again to the boulder,
and shall now seek to learn the lessons which the sand and pebbles have
to teach us. Their subject belongs to what is called physical geology,
and will bring before us some of the more mechanical operations of
nature, such as the slow but constant action of air, rain, and rivers,
upon hard rock, the grinding action of the waves, and the consequent
accumulation of new masses of sedimentary rock.
In all the departments of nature that come under the cognizance of man,
there is seen to be an endless succession of mutations. According to
the Samian philosopher--
"Turn wheresoe'er we may by land or sea,
There's nought around us that doth cease to be.
Each object varies but in form and hue,
Its parts exchange; hence combinations new.
And thus is Nature through her mighty frame
For ever varying, and yet still the same."
In the world of life we see how animals are sustained by a constant
series of chemical changes in their blood, every respiration of air
adding, as it were, fresh fuel to the flame of life within. In plants,
too, there is an analogous process. The atmospheric air is by them
decomposed, part of it being given off again, and part retained to
build up the organic structure. Plants withdraw mineral matter from the
soil, animals feed upon plants, and thus the earthy substances, after
having formed a part, first of rock masses, then of vegetable, and
subsequently of animal organizations, are returned again to the soil,
whence to be once more withdrawn and undergo new cycles of mutation.
But this perpetual interchange is not confined to the vital world. We
see it in the action of winds, when heated air rises and moves in one
direction, and the colder parts sink and travel the opposite way. The
same principle is exhibited by the oceanic currents, the removal of a
body of water, from whatever cause, always necessitating the ingress of
a corresponding quantity to supply its place. But perhaps one of the
most beautiful instances of these interchanges in the whole inorganic
world is the ceaseless passage of water from the land to the sea, and
from the sea to the land. The countless thousands of rivulets, and
streams, and gigantic rivers, that are ever pouring their waters into
the great deep, do not in the least raise its level or diminish its
saltness. And why? Simply because the sea gives off by evaporation
as much water as it receives from rain and rivers. The vapour thus
exhaled ascends to the upper regions of the atmosphere, where it forms
clouds, and whence it eventually descends as rain. The larger part of
the rain probably falls upon the ocean, but a considerable amount is
nevertheless driven by winds across the land. This finds its way into
the streams, and so back again to the sea, only, however, to be anew
evaporated and sent as drizzling rain across the face of land and sea.
This interchange is constantly in progress, and seems to have been as
unvarying during past ages.
But the ceaseless passage of water between land and sea is not a mere
isolated and independent phenomenon. Like all the rest of Nature's
processes, even the simplest, it produces important and complicated
effects. And the reader may, perhaps, think it worth looking at for a
little, when he reflects that to this seemingly feeble cause we owe no
small part of our solid lands, whether as islands wasted by the sea,
or as part of vast and variegated continents, wide rolling prairies
covered with verdure and roamed over by herds of cattle, or wintry
Alpine hills lifeless and bare.
The truth of this will appear when we reflect that the moisture which
rises from the sea and falls on the land as rain, is free from any
admixture of impurities; but by the time it again reaches the sea,
after a circuit of perhaps many miles down valley and plain, it has
grown turbid and discoloured, carrying with it a quantity of mud, sand,
and drift-wood. The sediment thus transported soon sinks to the bottom,
where it eventually hardens into rock, and in course of time is raised
above the waves as part of a new land. Such I conceive to have been the
origin of the sand, gravel, and imbedded plants of our boulder. It may
be well, however, in going into the details of the subject, to take a
wider view of this interesting branch of geology, and look for a little
at the forms and modes of the decomposition of rocks, and the varied
manner in which new sedimentary accumulations are formed.
All over the world, wherever a land surface spreads out beneath the
sky, there goes on a process of degradation and decay. Hills are
insensibly crumbling into the valleys, valleys are silently eroded,
and crags that ever since the birth of man have been the landmarks of
the race, are yet slowly but surely melting away. It matters not where
the hill or plain may lie, the highest mountains of the tropics and
the frozen soil of the poles, yield each in its measure and degree
to the influence of the general law. It might seem that so universal
a process should be the result of some equally prevalent and simple
cause. But when we set ourselves to examine the matter, we find it far
otherwise. The waste of the solid lands, in place of arising from some
single general action, is found to result from a multiform chain of
causes, often local in their operation and variable in their effects.
Such an investigation affords a good illustration of the general mode
and fashion in which Nature delights to work. It shows us that what
may seem a very simple process may be in reality a very complicated
one; that in truth there exist in the world around us few if any
simple, single processes, which stand out by themselves unconnected
with any other; that, on the contrary, all become intimately linked
together, the effects of one often forming part of the chain of causes
in another, and producing by their combined action that complex yet
strikingly harmonious order that pervades all the operations of Nature.
To an extent of which Cicero never dreamed, there runs through all
the world "such an admirable succession of things that each seems
entwined with the other, and all are thus intimately linked and bound
together."[51] Man separates out these various processes, classifies
and arranges them, because from the imperfection of his mental powers
he cannot otherwise understand their effects; all would seem but
chaos and confusion. But the formal precision and the sharp lines of
demarcation exist only in his mind. They have no place in the outer
world. There we see process dove-tailing with process, and spreading
out over the material world in an endless network of cause and effect.
We feebly try to trace out these interlacing threads, but we can follow
them far in no direction. Proteus-like, they seem to change their
aspect, blending now into one form, now into another, and so eluding
our keenest pursuit.
[Footnote 51: Cicero, _De Nat. Deor._ lib. i. 4. So, in Bacon's _Wisdom
of the Ancients_, under the fable "Pan or Nature:"--"The chain of
natural causes links together the rise, duration, and corruption; the
exaltation, degeneration, and workings; the processes, the effects,
and changes, of all that can any way happen to things." Such is the
philosopher's explanation of the Destinies as sisters of Pan. In no
part of his writings can the thorough practical character of Bacon's
philosophy be more conspicuously seen than in his treatment of these
ancient fables. Glancing over the titles of the different papers, you
are tempted to wonder what an intellect which could only appreciate
poetry as a mode of narrating history or as a vehicle for the teaching
of truth, will make of such fairy tales as those of Pan, Orpheus,
Proteus, Cupid, and many others. They seem like so many airy Naiads
crushed within the iron grasp of a hundred-banded Briareus. But a
perusal of those delightful pages will show that the giant has really
no malevolent intentions towards his fair prisoners; nay, that he only
wishes, by stripping them of their paint and finery, to show that,
with all their lightness and grace, they are nevertheless strong buxom
dames, of the same doughty race with himself.]
As an instance, therefore, of this remarkable interlacing of different
causes in what we call a single process, the disintegration of rocks
deserves our attention. In ordinary language, we say a stone rots
away, and its debris is washed down by the rains and streamlets, and
the process does not at first sight seem at all more complex than the
expression used to describe it; yet if we examine the subject, we shall
ere long find that there are in nature many simpler things than the
rotting away of a stone. To effect such a result, there come into play
a whole category of agencies, chemical and mechanical, so combined in
their operation, and so intimately blended in their effects, that it
becomes no easy task to tell where one set ends and another begins.
A rock is said to undergo a chemical change, when one or more of its
component parts passes from one state of combination to another--as,
for instance, when a mineral absorbs oxygen, and, from the condition
of a protoxide, changes into that of a peroxide; or when, parting with
its silicic acid, it takes an equivalent amount of carbonic acid, and
in place of a silicate becomes a carbonate. Now these, and similar
metamorphisms, are chiefly produced by water permeating through the
rocky mass, and thus no sooner does the old combination cease, than the
new one which replaces it is dissolved by the slowly filtering water,
and carried away either to greater depths, or to the surface. Every
drop of water, therefore, that finds its way through the rock, carries
away an infinitesimal portion of the mineral matter, and the stone is
consequently undergoing a continual decay. This condition of things may
go on either at some depth in the earth's crust, or on the surface.
In the former case, springs and percolating water are the agents in
effecting the change; in the latter, it is produced chiefly by rain and
streams. But wherever the process goes on, the results, unless where
counteracted by some opposite agency, are ultimately the same. It may
be of use to look at some examples of these changes, and, by dividing
them in a rough way, into underground and surface actions, we shall be
enabled to mark more clearly their effects.
A common source of the decay of rocks arises from the percolation
through them of water charged with carbonic acid. Decomposing
vegetation gives off a large amount of this gas, which is readily
absorbed by rain-water. The water sinks into the ground filtering
through cracks and fissures in the rocks, whence it afterwards
re-emerges in the form of springs. Now wherever, in its passage through
these subterranean rocks, the water meets with any carbonate, the
carbonic acid contained in the liquid immediately begins to dissolve
out the mineral matter, and carries it eventually to the surface. There
the amount of evaporation is often sufficient to cause a re-deposit of
the mineral in solution. If it be lime, a white crust gathers along
the sides of the stream, delicately enveloping grass-stalks, leaves,
twigs, snail-shells, and other objects, which it may meet with in its
progress. Such "petrifying" springs, as they are popularly termed,
occur abundantly in our limestone districts. It should be borne in
mind, however, that they only produce an incrustation round the organic
nucleus, and do not petrify it. That alone is a true petrifaction
where the substance is literally fossilized, or turned into stone.
A familiar instance of a similar chemical process may be seen under
many a bridge, and along the vaulted roofs of many an old castle.
Numerous tapering stalactites hang down from between the joints of the
masonry, resembling, so to speak, icicles of stone, often of a dazzling
whiteness. They are formed by the percolation of carbonated water
through the mortar of the joints, the carbonate of lime thus withdrawn
being re-deposited where the water reaches the air and evaporates. A
little pellicle of lime first gathers on the roof, and every succeeding
drop adds to the length of the column. In some cases, where the supply
of water is too great for the amount of evaporation, part falls on the
floor, and, being there dissipated, leaves behind a slowly-gathering
pile of lime called stalagmite. In some of the Eastern grottos, the
pillars from the roof have become united to those on the floor, forming
the most exquisite and fairy-like combinations of arch and pillar.
An example of a calcareous grotto has now become pretty familiar to
our summer tourists, under the name of the Spar Cave. It lies on an
exposed cliff-line along the western shores of Skye, against which
the surge of the Atlantic is ever breaking. You approach it from the
sea, and enter a narrow recess between two precipitous walls of rock,
open above to the sky, and washed below by the gurgling tide. Crossing
the narrow, shingly beach, you find the ground thickly covered with
herbage, while, grouped along the dark walls, are large bunches of
spleenwort, hart's-tongue fern, and other plants that love the shade.
Soon after entering the cave, all becomes sombre and cold; and the few
candles, with which the party have furnished themselves, only serve
to heighten the gloom. After scrambling on for a time across dank,
dripping rocks, and over a high bank of smooth marble, on which it is
difficult to creep, almost impossible to stand, you arrive at a deep
pool of clear, limpid water, which extends across the cave from side to
side, barring all farther passage. The scenery at this point will not
readily be forgotten. The roof towers so high that the lights are too
feeble to show it, while the walls, roughened into every form of cusp
and pinnacle, pillar and cornice, all glittering in the light, resemble
the grotto of some fairy dream. On returning again to the light of day,
if you ask the cause that has given rise to all this beauty, it will
be found a very simple one. The cleft occupied by the cave has been
once filled by a wall of igneous rock called a trap-dyke. Atmospheric
influences, aided probably by the waves, have caused the decomposition
and removal of this intruded rock, and the calcareous sandstone on
either side 'now stands up in a wall-like form. The upper part of
the dyke remains as a roof to the cave, but it has become completely
covered over with the calcareous deposits left by the carbonated water
that filters through the adjacent limy sandstone. The amount of water
is considerable, and consequently every part of the cavern--roof,
walls, and floor--has been incrusted with a white crystalline carbonate
of lime. In volcanic countries, where the springs often come to the
Surface in a highly heated state, charged, too, with various chemical
ingredients, they produce no slight amount of physical change on the
surrounding districts, and must be regarded as important geological
agents.
But perhaps the most common and widely-diffused form of decomposition,
is that produced on the surface of the earth by the action of
rain-water, in slowly dissolving out the soluble parts of rocks, and
washing away the loose, incoherent grains that remain behind. It is
hard to say whether this process is more chemical or mechanical.
The solution of the mineral matter belongs to the former class of
changes, while the removal of debris must be ranked among the latter.
The results of these combined forces form one of the most important
branches of investigation which can occupy the attention of the
physical geologist, and in contemplating them, we are at a loss whether
most to admire their magnitude, or the immense lapse of time which
they must have occupied. It may be worth while to look at the progress
of this kind of disintegration, that we may see how wide-spread and
constant is the waste that goes on over the world, and how materially
the effects of running water are by this means increased. A volume
might be written about the decay of rocks, and a most interesting one
it would be, but its authorship would devolve rather on the chemist
than the geologist.[52] We can do no more here than merely glance at
one or two illustrative examples.
[Footnote 52: A German chemist, Bischoff by name, has written two
learned volumes in which this subject is discussed (translated into
English, and published by the Cavendish Society), valuable for their
facts, but not always very safe in their deductions.]
Among the mineral substances that most readily yield to the action of
the weather, are the silicates and the carbonates. The rocks containing
the former belong in large measure to what we call the igneous class,
such as the granites and traps; while those containing the latter
form the bulk of our useful stones, such as limestone and sandstone.
The removal of alkaline silicates is due to their conversion into
carbonates, which are readily soluble in water. Rain falling on a rock
in which they are largely present, dissolves a small portion, and
carries it into the soil or into streams, and thence to the ocean.
Every shower in this way withdraws a minute amount of mineral matter,
and tends to leave the harder insoluble grains of the rock standing out
on the surface in the form of a loose pulverulent crust, easily washed
away. The debris thus formed, where allowed to accumulate, makes an
excellent kind of soil known to the Scottish farmers as "rotten rock."
The tourist who has visited any of our granitic districts, such as the
south-western parts of Cornwall, the rugged scenery of Arran, or the
hills of the Aberdeenshire Highlands, must be familiar with some of the
forms of waste which the rocks of these regions display. Mouldering
blocks, poised sometimes on but a slender base, and eaten away into the
most fantastic shapes, abound in some localities, while in other parts,
as for instance at the summit of Goatfell in Arran, the rock weathers
into a sort of rude masonry, and stands out in its nakedness and ruin
like some crumbling relic of Cyclopean art. In other districts, as in
Skye and in the adjoining island of Raasay, the granitic hills are
of a still more mouldering material. Their summits, white and bald,
sometimes rise to a height of fully two thousand feet above the sea,
while down their sides are spread long reddish-yellow tracks of debris
intermingled with patches of stunted herbage. Every winter adds to the
waste, and lengthens the lines of rubbish. Some of these hills form a
good field wherein to study the disintegration of granitic rocks, such,
for instance, as Beinn na Cailleaich, that rises from the shores of
Broadford Bay. Around the eastern base of that mountain there stretches
a flat moory district, with a few protruding blocks that have rolled
down into the plain. The earlier part of the ascent lies over a region
of metamorphic limestone, where the grey weathered masses of the
calcareous rock, often like groups of mouldering tombstones, are seen
protruding in considerable numbers through the rich soft grass and the
scanty brushwood of hazel and fern. Leaving this more verdant zone, we
enter a district of brown heath that slowly grows in desolation as we
ascend. Huge blocks of syenite--a granitic rock of which the upper part
of the mountain entirely consists--cumber the soil in every direction,
and gradually increase in numbers till the furze can scarcely find a
nestling-place, and is at last choked altogether. Then comes a scene
of utter desolation. Grey masses of rock of every form and size are
piled upon each other in endless confusion. Some of them lie buried in
debris, others tower above each other in a rude sort of masonry, while
not a few perched on the merest point seem but to await the storms of
another winter to hurl them down into the plain. The ascent of such a
region is no easy task, and must not unfrequently be performed on hands
and knees. But once at the top, the view is enough to compensate a
tenfold greater exertion. Far away to the west, half sunk in the ocean,
lie the isles of Eigg, Coll, and Tiree, with the nearer mountains of
Rum. North-west, are the black serrated peaks of the Coolins, that
stand out by themselves in strange contrast with every other feature
of the landscape. Northward, stretches the great range of syenitic
hills, with the sea and the northern Hebrides beyond. Away to the east,
across the intervening strait, lie the hills of the mainland, with
all their variety of form and outline, and all their changing tints,
as the chequered light and shade glide athwart the scene. Southward,
the eye rests on the grey wrinkled hills of Sleat, and far over along
the line where earth and sky commingle, are the mountains of Morven,
stretching westwards till they end in the bold weather-beaten headland
of Ardnamurchan, beyond which lies the blue boundless ocean. The top of
Beinn na Cailleaich is flat and smooth, surmounted in the centre by a
cairn. Tradition tells that beneath these stones there rest the bones
of the nurse of a Norwegian princess. She had accompanied her mistress
to "the misty hills of Skye," and eventually died there. But the love
of home continued strong with her to the end, for it was her last
request that she might be buried on the top of Beinn na Cailleaich,
that the clear northern breezes, coming fresh from the land of her
childhood, might blow over her grave.
I have already alluded to the wasting away of a trap-dyke. This
decomposition arises from the same cause as among the granites--the
solution, and removal of the silicates. All these trap-rocks are
igneous, and seem to have risen from below through open fissures and
rents. As they contain a large percentage of felspar--the same mineral
that gives to many granites their mouldering character--they may be
seen exhibiting every form and stage of decay. Often they stand out in
prominent relief from some cliff of soft shale, with a brown surface,
picturesquely roughened into spherical masses of all sizes, that give
to the rock somewhat the appearance of a hardened pile of ammunition
in which ponderous shells lie intermingled with round shot, grape, and
canister. Each of the concretionary balls when examined is found to
exfoliate in concentric pellicles like the coats of an onion, and you
may sometimes peal off a considerable number before arriving at the
central core, which consists of the hard rock still undecomposed. In
this case the process of degradation is aided by the decay of another
mineral called augite, which contains a variable percentage of iron,
and imparts the peculiar yellowish-brown tint to the weathered rock.
Trap-dykes may also be seen in a still more wasted condition, where,
in place of protruding from a cliff-line, they recede to some depth
and give rise to deep clefts and fissures. An instance of this kind
has been referred to in the case of the Spar Cave, and many others may
be seen along the same coast-line. The shore there for miles is formed
of a low cliff of white calcareous sandstone, fissured by innumerable
perpendicular clefts of greater or less width, and sometimes only a
yard or two apart. Each of these has once been filled by a dyke of
trap, which originally rose up in a melted state, and after having
solidified into a compact stony mass, began to yield to the process of
decay. In all these and similar cases, the primary cause of the waste
lies in the decomposition of the felspar. Rain-water acts in removing
the soluble portions, and the harder grains that remain, deprived of
the cementing matrix, ere long crumble down and are washed off by the
rains. In this way the rock insensibly moulders away, every frost
loosening its structure, and every shower carrying away part of its
substance.
Among the many objects of interest along a rocky coast some of the more
striking are certainly to be found in the curious and often grotesque
forms assumed by the weathered cliffs. Above high-water mark and thus
away from the dash of the waves, we can often trace the progress of
decay among such sedimentary rocks as sandstones and conglomerates.
Worn into holes and scars, projecting cusps and tapering pinnacles, or
eaten away into the rude semblance of a human form, headless perchance,
or into the shape of a huge table poised on a narrow pedestal, the
rock affords an endless variety of aspects and a continual source of
pleasure. If we chance to light upon any building constructed out of
the sandstone of such cliffs, it is worth noting that the removal of
the stone has not deprived it of its mouldering qualities; nay, that
houses erected within the memory of people still living already begin
to wear an aspect of venerable antiquity. I remember meeting with an
interesting example in the case of an old castle built on a similar
rocky coast-line. It stood on a little ness or promontory of dull red
stone, washed on all sides save one by the wild sea. The walls, of
which but a fragment remained, were built of a dark red sandstone; but
the lapse of centuries had told sadly on their masonry. The stones
rose over each other tier upon tier, corroded sometimes into holes
and hollows, sometimes into a close honey-combed surface, but the
mortar that had been used to cement them together still stood firm and
protruded from between the tiers to show, by no doubtful or ambiguous
sign, how silently yet how surely the wasting forces had been at
work. The scutcheon over the only remaining gateway had been carved
out of another kind of stone of a lighter colour and harder texture,
and so its grim lions looked nearly as fresh and formidable as when
first raised to the place of honour which they still occupied. In
this case, as before, the decomposition was owing to the presence of
a considerable proportion of soluble matter, which the rains of four
centuries had carried away along with the loosened incoherent sandy
grains.
Conglomerate or pudding-stone has often a picturesque outline in its
decay, more especially if its included fragments have a considerable
range of size. Large tracts Of this rock exist in various parts of
Britain, particularly in Scotland, where the basement beds of the Old
Red Sandstone consist of a coarse conglomerate, sometimes several
thousand feet in thickness. Such enormous masses form the scenery of a
large part of East Lothian, and are found in detached patches across
into Peeblesshire and Lanark. In the north, too, the neighbourhood of
Inverness and other parts of the same district display conspicuous
conglomerate hills. Unless where laid bare by streams or by the action
of breakers, the contour of these hills is rounded and tame, with a
scanty covering of short scrubby grass and very few protruding bosses
of rock. But where a mountain torrent has cut its way down the hill
side, the ravine thus formed exhibits broken walls and pillars of
rock made up of rounded balls of every shade and size, cemented by a
dark-red or green paste. The cementing material is sometimes clay,
sometimes lime, and its variable nature gives rise to a corresponding
inequality in the amount and form of decomposition. Where the rounded
pebbles are bound together by clay, rains act with rapidity in washing
away the cement, and the component balls fall out by degrees, leaving
a cliff strangely roughened by protruding knobs, and eaten away into
clefts and hollows. When the pebbles are held in a crystallized matrix
of lime, they usually remain longer together, and may sometimes be
seen standing up in the form of detached rugged pillars that defy
all regularity of size or outline, and remind one of a sort of rude
grotto-work. Such irregularities become still more marked where to the
action of the rains there has been added the spray of the ocean. A
coast-line of conglomerate, where the rock rises into cliffs, is always
a romantic one; caves, pillars, and ruined walls, all in the same rough
grotto style, meet us at every step. Here, too, we can mark the varying
effect of the waves upon the lower portions of the rock, eating it into
cavernous holes and leaving rugged projecting pinnacles to which the
mottled colours of the included pebbles give an additional and peculiar
effect.
A cliff of shale seldom shows much of the picturesque, though often a
good deal of the ruinous. The rock is easily undermined by streams,
and a shale ravine usually exhibits in consequence either heaps of
crumbling rubbish, or, where the stream comes past with a more rapid
current, perpendicular walls, jointed and laminated, but without much
variety of outline. Such cliffs, however, merit the careful attention
of the observer, for from their friability they are most easily
decomposed and washed down by streams, to form new accumulations of
similar soft argillaceous matter. A shale coast-line sometimes shows
cliffs of considerable altitude, as in some parts of Skye and Pabba,
where the Lias shales may be seen piled over each other often to a
height of seventy or eighty feet, and spreading out along the shore as
low flat reefs and skerries, brown with algæ at their seaward ends,
and showing on the higher slopes of the beach the characteristic
fossils of the Lias--_ammonites_, _belemnites_, and _gryphææ_--crowded
together by hundreds. The action of the decomposing forces has operated
more effectually on the soft material of the shale than on the hard
crystalline lime of the included shells, so that the latter stand out
in relief from the dull-brown surface of the rock, and from their
numbers and prominence form one of the most marked features of the
coast-line.
Probably few have ever visited a limestone district without marking
the manner in which that rock yields to the action of the elements,
whether in an inland part of the country where rivers have cut deep
gullies through the rock, or along some exposed shore where the stone
has been wasted by a still ruder assailant. An exposed cliff of hard
homogeneous limestone weathers into deep clefts and holes; the entire
surface assumes a pitted appearance, somewhat like a sandy beach after
a showier of rain, and the planes of stratification, or lines formed
by the parallel junction of the beds are often worn away until the
rock looks not unlike a piece of old masonry, in which the mortar has
decayed and dropped out, leaving the angles of the stones to get wasted
and rounded by the action of the weather. In many districts, too, where
the rock is richly fossiliferous, the broken joints of encrinites along
with corals and shells may be seen crowded together by myriads, their
hard skeletons protruding from the wasted rock in such a way as to show
that the stone can contain very little else. By this means we often
learn that a limestone bed is nothing but an old sea-bottom, where
the calcareous sediment was mainly derived from broken stone-lilies,
corals, and shells, though if we break off a piece of the rock the
internal fracture may show very little or no trace of any organic
structure. And hence if the geologist would form an accurate conception
of the origin and structure of many of the stratified rocks, he must
study them not in hand specimens neatly trimmed and arranged along the
shelves and drawers of a cabinet, nor even in the ponderous blocks
daily exhumed by the quarryman, but along some surf-beaten cliff-line
or down some precipitous ravine where the rock for centuries has been
exposed to the wear and tear of the elements.
Limestones and other calcareous formations are liable to more than
ordinary decay, for, as we have seen above, percolating rain-water
constantly carries away mineral matter from their subterranean
portions. Accordingly, in some parts of the country, as for instance in
Yorkshire, the interior of such rocks has been eaten away into great
caverns by this form of decomposition.
Some remarkable examples occur in the island of Raasay, one of the
north-eastern Hebrides. Its eastern margin shoots up from the sea to a
height of over 900 feet, the cliff-line being formed of a calcareous
grit as perpendicular as a wall, and fissured by deep chasms and rents.
The narrow table-land between the edges of this cliff and an abrupt
ridge that rises behind, is perforated by innumerable holes and clefts,
into which if a stone be thrown it may be heard for several seconds
rumbling far below. The edges of these pitfalls are often fringed
with ferns, rushes, and long grass, so as to be nearly hidden, and it
requires no little caution to traverse this elevated region in safety.
Innumerable sheep have been lost by falling into the subterranean
abysses, and even the wary natives seem to have sometimes lost their
footing. A story is told of a woman who had crossed to the other side
of the island for the purchase of some commodities, and returning
by the high grounds had got nearly within sight of her own cottage,
when by some unlucky accident she took a false step and instantly
disappeared. Unfortunately her errand had been performed alone, so that
some time elapsed ere she was missed. The search continued unremitting
for two days, but no trace of the missing traveller could be found. At
last on the third day her figure was seen creeping slowly along the
road not many hundred yards from her own door. It appeared that she had
first slid down a sheer height of about fifty feet, when her further
passage was intercepted by the sides of the fissure. During the earlier
part of her confinement she strove hard to re-ascend the chasm, and it
was not until, the effort seeming fruitless, she had begun to resign
herself to despair, that a glimmering of light from below induced her
to attempt a descent. This proved no easy matter, and occupied many
weary hours of labour and suspense; but at length she succeeded in
worming herself to the bottom, and crawled out more dead than alive
only a little way from her home. There still stand perched on some of
these precipitous cliffs the remains of a few villages, the inhabitants
of which were accustomed to tether their children to the soil, whence
one of the hamlets received in Gaelic the soubriquet of Tethertown.
Many a valuable commodity disappeared by rolling over the cliff, and
I have been assured that it was no unfrequent occurrence for a pot of
potatoes capsized at the doorway to tumble down the slope and make no
stop until safely esconced at the sea-bottom.
The process whereby these fissures and caverns originate is the same as
that noticed already in the Spar Cave. Water containing an impregnation
of carbonic acid filters down through cracks and fissures of the
calcareous rock, dissolving out in its passage a portion of the lime
which it eventually carries back to the surface, and either deposits
there or transports into streams, and thence to the sea. Thus atom
by atom is removed wherever the percolating water reaches, until in
the course of ages an irregular cavern of greater or less extent is
produced. The decomposition of limestone at the surface results from
the same kind of action, that of carbonated water. Every shower of
rain insensibly carries away a fraction of the constituent parts of
the rock, so that the size and form of detached blocks as well as of
exposed cliffs is constantly changing. How often do we see the same
decay going on with a melancholy rapidity among the exposed marble
tombstones of our churchyards. In a few years the tablet gets worn
and furrowed as though it had stood there for centuries. Eventually,
too, the inscription becomes effaced, and perhaps ere the bones of the
deceased have mouldered away and mingled with their kindred dust, the
epitaph that recorded for the admiration of posterity his many virtues
and his vigorous talents, has faded from the stone--often, alas! only
too fit an emblem of how speedily the memory of the dead may fade away
out of the land of the living.
CHAPTER IX.
Mechanical forces at work in the disintegration of rocks--Rains
Landslips--Effects of frosts--Glaciers and icebergs--Abrading
power of rivers--Suggested volume on the geology of
rivers--Some of its probable contents--Scene in a woody
ravine--First idea of the origin of the ravine one of primeval
cataclysms--Proved to be incorrect--Love of the marvellous
long the bane of geology--More careful examination shows the
operations of Nature to be singularly uniform and gradual--The
doctrine of slow and gradual change not less poetic than that
of sudden paroxysms--The origin of the ravine may be sought
among some of the quieter processes of Nature--Features of
the ravine--Lessons of the waterfall--Course of the stream
through level ground--True history of the ravine--Waves and
currents--What becomes of the waste of the land--The Rhone
and the Leman Lake--Deltas on the sea-margin--Reproductive
effects of currents and waves--Usual belief in the stability
of the land and the mutability of the ocean--The reverse
true--Continual interchange of land and sea part of the
economy of Nature--The continuance of such a condition of
things in future ages rendered probable by its continuance
during the past.
The forms of decomposition noticed in the last chapter were chiefly
of a chemical kind. Their effects were observable alike on the
surface of the earth and below ground; in the latter case we saw them
excavating caverns and long irregular chasms, in the former we noted
the production of debris which if undisturbed went to the formation
of soils. It must be borne in mind however, that in these operations
other forces than simply those of a chemical kind come into play. The
percolation of water and the removal of insoluble particles on the
exposed parts of rocks rank as mechanical processes. So also do those
by which new surfaces of mineral masses are brought within the sphere
of the chemical agencies, such as the action of frosts, rains, rivers,
and waves. In short, as already noticed, any subdivision of the forces
at work in effecting the decomposition of rocks must ever be more or
less arbitrary; but it remains nevertheless useful, if we bear in
mind that the exactly defined boundary lines are of our making, not
Nature's. With this caution we may proceed to examine what are termed
the mechanical agencies in the disintegration of mineral masses, and in
so doing, we shall find that the chemical forces are not less helpful
to the mechanical than the latter to the former.
First, we may notice the effect of rains in washing away the
disintegrated particles to lower levels or into river-courses whereby
fresh portions of rock become exposed to the decomposing forces. Rains
also act powerfully in altering the form of cliff-lines and steep
declivities, especially where these consist more or less of friable
earthy matter. After a long continuance of wet weather, I have seen
the abrupt sides of a river-channel that were formed of a stiff blue
clay completely cut up by rents of various dimensions, whereby large
masses had subsided many feet, while others had rolled down altogether
and lay in the bed of the stream where they were undergoing a rapid
abrasion. The cause of this alteration was obvious. The rains pouring
down from the sloping grounds on either side of the river had excavated
deep channels on the abrupt face of the cliffs, while a considerable
quantity of water finding its way through the soil, had permeated
through joints and crevices in the clay some feet from the edge of the
bank. By the combined operation of these causes, masses of clay several
yards in extent lost their cohesion and either settled down a few
feet, or found their way to the bottom. Such landslips are of frequent
occurrence where large masses of rock of a hard compact nature rest
upon loose shales and clays more or less inclined. Whole hills have
been known to be hurled in this way into the valleys below.
But these results become perhaps still more marked where to the
ordinary operations of water there are added those of intense frost.
The effects of a severe winter (such, for instance, as a Canadian one),
in loosening the particles of rocks and facilitating the breaking-up
of large masses, must be ranked among the most powerful agencies of
nature. In such a season, the percolating water with which nearly every
surface-rock is charged becomes frozen, and in the act of congelation
expands. The result of this dilatation is to exert great pressure on
the particles of the rock, and thereby loosen their cohesion. When thaw
comes the frozen liquid contracts again, but the loosened particles
have no such elastic power, and so, having lost hold of each other,
crumble down. If the season be a changeable one, frost and thaw quickly
alternating, the amount of waste produced becomes very great. Not only
is the outer surface of the stone decomposed, but the water filtering
through the joints of the rock freezes there, and thus on the arrival
of milder weather vast masses become detached from the cliffs, and
roll down, to be worn by the grinding action either of waves or of
rivers, as the case may be. Spring at last sets in with its warmth and
its showers; the snow rapidly melts away; the whole country streams
with water; every valley and hollow has its red turbid rivulet, that
bears a burden of muddy sediment into the nearest river; and thus the
loosened portions of the rocks get washed away down to sea, leaving a
new surface for the action of next winter. We can easily understand,
therefore, that in certain regions the combined effects of frost and
thaw may work in the course of ages changes of almost inconceivable
extent, and that the agency of ice must be not less varied and
important on the land than, in the case of the boulder clay, we found
it to be in the ocean.
Besides this action in winter, which goes on more or less in every
country wherever the temperature sinks sufficiently low to permit of
the freezing of water, ice effects many changes on the surfaces of
rocks when it takes the form of glaciers and icebergs. We have already
noted the operation of a glacier during its slow progress in crushing
down large fragments of stone, scratching and abrading the rocks over
which it passes, and eventually producing a vast quantity of mud, which
is carried down by streams to form new accumulations either in lakes
or seas. We have also marked the effects of the drifting iceberg in
materially modifying the contour of submarine hills, and depositing
over the ocean-bottom mud, gravel, and boulders. Nothing further,
therefore, need be done here than simply to keep these agencies in
view, as playing an important part in the disintegration of rocks.
Another highly interesting aqueous action is that of streams and
rivers, in scooping out for themselves channels through sometimes
the hardest and most solid rock. Such effects may be seen all over
the globe, in the old world and in the new, in the bed of the
tiniest rivulet, as well as in the course of the mightiest river.
And accordingly, in all the long list of geological agents, we find
none so well known and so often described alike by poets, historians,
and scientific writers, as well in ancient as in modern times. What
a delightful volume might be written about the geology of rivers! It
would, perhaps, begin with that "great river," the Euphrates, along
whose green banks lay the birthplace of the human race, tracing out the
features of its progress from the ravines and cataracts of Armenia,
with all their surrounding relics of ancient art, down into the plains
of Assyria, amid date-palms and Arab villages, onwards to the mounds
of Nineveh and Babylon, and thence to the waters of the Persian Gulf.
Well-nigh as remote, and perhaps still more interesting in its human
history, would be the story of the Nile. We should have to follow that
river from the mystic region of its birth,[53] marking the character
of the rocks through which winds its earlier channel, and the effects
upon them of the floods of untold centuries; it would be needful, too,
to note the influence of the waters on the lower grounds, from where
the stream flows over the cataracts of Syene, down through the alluvial
plains of Egypt; and lastly, the concluding and perhaps most onerous
part of our labour would be the investigation of the delta, marking its
origin and progress, its features in ancient times, as made known to us
in the graphic chapters of Herodotus, and the changes which the lapse
of more than twenty centuries has since wrought in its configuration.
The rivers of Europe would detain us long, not less perhaps by their
historic interest than by the variety and attractiveness of their
physical phenomena. One could scarce help lingering over the Rhine,
with its source among Alpine glaciers, its lakes and gorges, its
castles and antique towns; and when once the narrative entered the
classic ground of Italy, it would perhaps become more antiquarian than
geological. The ravine of Tivoli, for instance, would certainly lay
claim to a whole chapter for itself, with its long-continued river
action, its ancient travertin, its beautiful calcareous incrustations,
and above all its exquisite scenery.
[Footnote 53: "Fontium qui celat origines Nilus" a description not less
true now than in the days of the Sabine bard.]
"Domus Albuneæ resonantis,
Et præceps Anio, ac Tiburni lucus, et uda
Mobilibus pomaria rivis."[54]
[Footnote 54:
"Albuna's grey re-echoing home,
And Anio, headlong in his foam,
And grove of Tivoli,
And orchards with their golden gleam,
Whose boughs are dipping in the stream
That hurries to the sea."
Hor. _Carm._ L vii. 12.]
And when could we exhaust all, that might be said about the rivers of
our own land?
"Of utmost Tweed, or Ouse, or gulfy Dun,
Or Trent, who, like some Earth-born giant, spreads
His thirty arms along the indented meads
Or sullen Mole, that runneth underneath;
Or Severn swift, guilty of maidens' death;
Or rocky Avon, or of sedgy Lee;
Or coaly Tyne, or ancient hallow'd Dee;
Or Humber loud, that keeps the Scythian's name;
Or Medway smooth, or royal-towered Thame."
Passing to the new world, a vast field would spread out before us: the
Mississippi, the Atchafalaya, the Ohio, the St. Lawrence, the Amazon,
and many other rivers that in some cases rise high among the regions
of perpetual snow, and after traversing large areas of country in the
temperate zone, fall into the waters of tropical seas. By studying such
examples of river-action and delta-formation as are presented by these
gigantic streams, we should arrive at some conception of the conditions
anciently at work in producing our present coal-fields. Nor would our
researches assume aught like completion until after a scrutiny of all
the larger and more important rivers of the globe. Such a work could
be undertaken, perhaps, only by another Humboldt. Its successful
accomplishment would certainly insure the highest renown to its author,
and incalculable benefits to science.
From what we have seen of the wide waste and decay everywhere in
progress on the solid lands of our planet, it becomes no difficult
matter to perceive what a number of agencies must be at work in the
formation of a river channel. Let the reader take his stand in some
wooded ravine, where the shelving rocks on either side are hung all
over with verdure, and a tiny streamlet murmurs on beneath with a
flow so quiet and gentle as scarcely to shake the long pendant willow
branches that dip into its surface, while the polished pebbles that
strew its bed lie unmoved by the rippling current that glides over
them. If in the midst of such a scene the question were to arise in
his mind, How came this deep, narrow ravine into existence? what
answer would in all likelihood be the first to suggest itself? His eye
would scan the precipitous walls of the dell, with their rocks cleft
through to a depth of perchance fifty feet. It would require no great
scrutiny to assure him that the beds on the one side formed the onward
prolongations of those on the other, and that consequently there must
have been a time ere yet the ravine existed, when these beds stretched
along unbroken. Satisfied with these results, his first impulse might
be to bethink him of some primeval earthquake, when the solid land
rocked to and fro like a tempested sea, and broke up into great rents
and yawning chasms. Into one of these clefts he might suppose the
little streamlet had eventually found its way, moistening the bare and
barren rocks, until at length their surface put on a livery of moss, or
lichen, or liver-wort, and the birch, the alder, and the willow, found
a nestling-place in their crevices. Such a view of the origin of the
woody dell would be certainly a very natural one, and in some instances
might be sufficiently correct, but in the present case it will not
explain the phenomena. If the reader will kindly permit me to visit the
locality in his company, perhaps we may be able to light upon the true
explanation, and see a few appearances worthy our attention.
First, then, how can we make sure that no convulsion of nature has
produced a rent in the rocks, and so helped the streamlet to a channel?
a simple question that may be well-nigh as simply answered. We stand
in the centre of the dell on a broad ledge of stone, round whose
well-worn sides the rivulet is ever eddying onwards. The block consists
of a pale sandstone lying in a bed about three feet thick, that dips
gently down the stream and underlies a seam of dull, soft, blue shale,
full of small shells. We trace the edge of this sandstone bed across
to the left-hand side of the ravine, and away up into the precipitous
cliff, till it is lost amid the ferns and brushwood. There can be no
doubt, therefore, that the ledge on which we were but now standing is a
continuous portion of the rocks that form the left side of the ravine.
Returning again to the centre of the stream, we proceed to trace out
the course of the other end of the same bed, and find that it, too,
strikes across to the rocks on the right-hand side without a break
or fissure, and passes up into the cliff, of which it forms a part.
Clearly, then, the sandstone bed runs in an unbroken, unfissured line,
from the one side to the other, and the rocks of either cliff form
one continuous series. There occurs no break or dislocation, which,
of course, there must have been had the ravine owed its origin to any
subterranean agency. And so we come to conclude that no great cataclysm
in primeval times, no yawning abyss, or gaping chasm, has had anything
whatever to do with the formation of our deep sequestered dell. What
then? "Whither shall we turn," you ask, "to find another agency equally
grand and powerful in its operation and mighty in its results?"
Stay, gentle reader. That craving for the grand and the sublime, that
hungering after cataclysms and convulsions, that insatiable appetite
for upheavals, and Titanic earth-throes, and all the mightier machinery
of Nature, has done no little mischief to geology. Men have reasoned
that gigantic results in the physical structure of the earth must
have had equally gigantic causes operating in sublime conflict and
in periodic paroxysms, now heaving a mountain chain to the clouds
of heaven, now swallowing up a continent in the depths of the sea.
Happily such extreme notions are fast passing away, though the old
tendency in a modified form still abounds. A closer scrutiny of Nature
as she actually shows herself, not as theorists fancy she should be,
has revealed to us that her operations are for the most part slow,
gradual, and uniform, and that she oftentimes produces the mightiest
results by combinations of forces that to us might seem the very
emblems of feebleness and inactivity. In place of sudden paroxysms
she demands only an unlimited duration of time, and with the aid of
but a few of these simple, tardy agents, she will eventually effect
results perchance yet more gigantic than could be accomplished even
by the grandest catastrophe. Nor in thus seeking to explain the past
by defining what seems the usual mode of Nature's operations in the
present, do we, as is sometimes alleged, deprive them of their high
poetic element. Assuredly there is something thrilling to even the
calmest imagination in contemplating the results of vast and sudden
upheavals, in picturing the solid crust of the earth heaving like a
ground-swell upon the ocean, in tracing amid
"Crags, knolls, and mounds, confusedly hurl'd,
The fragments of an earlier world;"
and in conjuring up visions of earthquakes, and frightful abysses from
which there ever rose a lurid glare as hill after hill of molten rock
came belching up from the fires below. But while far from denying that
such appearances may have been sometimes seen during the long lapse
of the geological ages, and that they give no little vividness and
sublimity to a geological picture, we claim for the doctrine of the
tranquil and uniform operation during past time of existing laws and
forces, an element not less poetic. In the former case the pervading
idea is that of unlimited expenditure of power, in the latter that of
unlimited lapse of time. In the one case the action is Titanic but
transient, in the other it is tranquil but immensely protracted. The
two doctrines in this way counterbalance each other; yet I cannot but
think that however impressive it may be to stand in some lone glen, and
while gazing at its dark jagged precipitous cliffs, to dream about the
paroxysmal convulsions of some hour far back in the distant past, the
scene becomes yet more impressive when we look on its nakedness and
sublimity not as the sudden and capricious creation of a day, but as
the gradual result of a thousand centuries. These cliffs may once have
been low-browed rocks rising but a little way out of a broad grassy
plain, and serving as a noon-tide haunt for animals of long extinct
races. Thousands of years pass away and we see these same rocks higher
and steeper in their outline, brown with algæ and ever wet with surf,
while around them stretches a shoreless sea. Ages again roll on, and
we mark still the same rocks shooting up as bleak crags covered with
ice and snow. Another interval of untold extent elapses, and rock,
snow, and ice have all disappeared beneath a broad ocean cumbered
with ice-floes and wandering bergs. Again the curtain drops upon the
scene, and when once more it rises, the cliffs stand out in much the
same abrupt precipitous aspect with that which they now present, save
that their bald foreheads look less seamed and scarred than now, and
their dark sides show no trace of bush or tree. The white cascades that
to-day pour down from their summits and sides--seeming in the distance
like the white hairs of age--are insensibly deepening the scars and
furrows on these ancient hills, and thus slowly but yet surely carrying
on the process of degradation and decay. Musing on all this long
series of stages in the formation of one single cliff-line, is there
not something more sublime, something yet more impressive than if we
pictured but the chance random result of the gigantic paroxysm of an
hour?
Let us not be deterred then from seeking an explanation of the origin
of the ravine among some of the quieter and more unobtrusive forces
of Nature. Give them but an unlimited period to work in and they will
abundantly satisfy all our demands.
We return again to the rocky ledge in mid-channel, and proceed to
ascend the course of the stream, marking as we go the changes in the
character and features of the stone that forms the cliff on either
hand. We come to a bare part of the ravine where brushwood and herbage
find but a scanty footing and where accordingly the rocks can be
attentively studied. The face of the escarpment shows a number of beds
of pale grey sandstone alternating with courses of a dark crumbly
shale. The sandstones being harder and firmer in texture stand out
in prominent relief while the shales between have been wasted away,
covering the bottom of the slope with loose debris. We can mark too
that, as this decay goes on, the harder beds continually lose their
support, cracking across chiefly along the lines of joint, and rolling
down in huge angular blocks into the stream. In truth we cannot doubt
that every year adds to this decay and thus slowly widens the dell, for
the broken fragments do not form in heaps over the solid rock below
so as to protect it from the weather, but are evidently carried away
by the stream and hurried down the ravine onwards to the sea. From
what has been said above relative to the disintegration of rocks by
percolating water, frosts, and other causes, the reader will easily
see how this rotting away of the sides of the ravine must be carried
on; and he will not fail to mark that here we have at work an agency
not yet considered, that of running water. The effects of the weather
are seen in the crumbling, ruinous cliffs overhead; the effects of
the streamlet are observable in the continual removal of the rubbish
whereby a fresh surface is ever exposed to the decomposing forces,
while at some points we can mark the water actually undermining an
overhanging part of the cliff from which there are ever and anon vast
masses precipitated into the channel where eventually they get worn
down and carried away out to sea. "Still," you may remark, "these
forces are at work only in widening a channel already made. How was the
ravine formed at first?"
We continue our ascent. A scrambling walk through briars and
hazel-bushes, sometimes on rocky ledges high among the cliffs,
sometimes among the prostrate blocks that dam up the stream, brings
us at last full in front of a sparkling waterfall that dashes over
a precipitous face of rock some twenty feet high. The appearances
observable here deserve a careful attention. Our eyes have not been
long employed noting the more picturesque features of the scene ere
they discover that the dark-brown band of rock forming the summit of
the ledge over which the water tumbles is continuous all round the
sides of the dell. There is consequently no break or dislocation here.
Approaching the cascade we note the rock behind it so hollowed out that
its upper bars project several feet beyond the under ones. In this
way the body of water is shot clear over the top of the cliff without
touching rock till it comes splashing down among the blocks in the
channel. And yet this hollowed surface is never dry; the spray of the
fall constantly striking on it keeps it always dank and dripping. In
some parts the rock stands out bare and worn, while on the less exposed
portions there gathers a thick green scum which is replaced on the
drier ledges by the soft cellular leaves of the liver-wort. Now our
examination of the influence of percolating water upon even the hardest
rocks teaches us that this moist soaked surface is just the very best
condition for favouring the decay of the rock. Nay more, the green
vegetation that mantles over the stone serves to prevent the water from
running off too rapidly, and keeps the rock in a still more moist state
than would otherwise happen. So that the portion of sandstone behind
the cascade comes to be in a still more favourable situation for speedy
decay than the ledge over which the water is rapidly driven. We can
see, therefore, how in the lapse of years the corrosion may go on until
the upper projecting part of the cliff loses its support and falls with
a crash into the rocky pool below, while the form of the waterfall
becomes thus greatly altered, and new surfaces are exposed to the wear
and tear of the stream.
But we have not yet exhausted all that the rocks at the cascade can
teach us. By dint of some exertion we climb the cliff and gain the
upper edge of the fall. The rocks that form the bed of the stream are
now seen to be deeply grooved and worn, every exposed surface having
a smoothed blunted aspect. We can mark how the stone has split up
along the natural lines of joint, whereby great facility is given to
the removing power of the current, and how large irregular angulated
blocks become detached and are swept down the stream. In not a few
parts, too, we may notice circular holes of greater or less depth, in
the bottom of each of which lie perhaps a pebble or two, that with a
constant gyratory movement, caused by the eddying water, have eaten
their way downwards into the solid rock. When the stream is in flood
and comes roaring down the rocky gorge bearing along with it a vast
amount of mud, gravel, and stones, one can easily see how the friction
of the transported material must wear down the hard bed and sides of
the channel, and how this process repeated month after month and year
after year, must aid the decomposing forces in scooping out a deep
ravine. From the cascade the ascent of the stream becomes steeper and
the run of water is consequently more rapid. Soon however we emerge
from the woody copse, and find ourselves on a flat alluvial cultivated
plain through which the rivulet winds in a tortuous meandering course,
bending back upon itself into loops that almost meet and well-nigh
form broad flat islets. Strolling along this winding route we can mark
the effects of the stream in eating away the soft clay and sand at one
part of the bend and piling them up at another. Such loose material
can present but little resistance to a stream swollen with rains, and
consequently a large quantity of the mud and gravel along with the
interspersed boulders must be swept away down into the dell at every
season of flood. The matter thus removed will of course be still
further comminuted in its passage, and at the same time will help to
grind down the hard rock surfaces over which it is driven.
Here then may be found the whole history of the ravine. Originally the
streamlet wound its devious course through a flat alluvial country with
a channel sunk but a foot or two below the level of the plain. Such
continued its character till it reached a low bluff, down which the
water flowed more rapidly to gain a second level undulating region.
The part of this bluff crossed by the stream was ere long bared of
its covering of soil and clay, and the rock below came to be washed
by a group of little cascades. Once exposed to the decomposing and
disintegrating forces, the stone soon began to decay and the cascades
ere long merged into one. By slow degrees the rock gave way and the
waterfall retreated from the bluff. For perchance thousands of years
the same process has been going on, now with greater, now with less
rapidity, according to the nature of the rocks encountered and other
modifying causes, until the fall has eaten its way back for well-nigh
three miles and scooped out a wild rocky gorge some fifty or sixty feet
deep. This is but a solitary and insignificant instance of what may be
seen all over the world, for the process remains the same whether we
stand beside a tiny rivulet in some lone Highland glen or listen to the
roar of the falls of Niagara.
There is but one other principal agency at work in the demolition of
rock-masses, the waves and currents of the ocean. But we have already
noted the effects thus produced, and need not now retrace our steps
further than to recall the vast amount of devastation which can be
shown to have been effected in our own country by marine causes, both
in breaching the existing shores and in scooping out valleys and
grinding down hills at former periods when the land was either rising
above or sinking below the level of the sea.
Having now satisfied ourselves that there goes on all over the world an
incessant waste of the solid lands, that the disintegrated debris is
washed down by rains and transported seawards by rivers, and that the
waves are ever eating their way into the iron-bound coast-line as well
as into the low alluvial shore, we naturally come to ask the result
and end of all this decay. What becomes of that vast amount of mineral
matter annually removed from the land? To be able to answer this
question clearly and distinctly, let us look for a little at what takes
place in lakes, at river-mouths, and in open sea.
The river Rhone rises among the Bernese Alps, and after a course of
about 100 miles through the Canton of Valais, it enters the upper end
of the Lake of Geneva. Its waters, where they mingle with those of the
lake, are muddy and discoloured, but where they pass out at the town of
Geneva are limpid and clear. The mud, therefore, which they bring into
the lake must be deposited there, and as the stream may have continued
to flow for thousands of years, we may reasonably expect to find some
trace of the large amount of sediment necessarily deposited during the
whole or part of that long period. Accordingly, careful examination of
the Lake of Geneva has shown that such accumulations have really been
formed, and that their progress and amount during part of the historic
period can be approximately calculated. Where the turbid current of the
Rhone enters the still water of the lake, the mud slowly sinks to the
bottom. In the lapse of centuries layer after layer has been thrown
down, rendering the lake at this part sensibly shallower, until a large
area or delta has been filled up and converted into a flat alluvial
plain. Thus, a town which in the time of the Romans formed a harbour
on the water's edge, now stands more than a mile and a half inland.
This new-formed land is entirely the work of the stream, and if we
could obtain a complete section of it from the surface to the bottom,
"we should see a great series of strata, probably from 600 to 900 feet
thick (the supposed original depth of the head of the lake), and nearly
two miles in length, inclined at a very slight angle." These strata,
which are said to have taken about eight centuries to form, "probably
consist of alternations of finer and coarser particles; for, during the
hotter months, from April to August, when the snows melt, the volume
and velocity of the river are greatest, and large quantities of sand,
mud, vegetable matter, and drift-wood, are introduced; but, during the
rest of the year, the influx is comparatively feeble, so much so that
the whole lake, according to Saussure, stands six feet lower."[55] If
the present conditions continue for a sufficient length of time, the
lake may be eventually filled up with mud, sand, and gravel, deposits
that would eventually harden by pressure into shale and sandstone. So
that the day may yet arrive when the blue waters of the Leman lake
shall have passed away, when the Rhone perchance may have ceased to
flow or found its way by some other channel, when the peasant may guide
the plough where now the boatman plies the oar, and when the geologist
shall trace out in quarries and excavations the successive deposits of
hardened sediment with their lacustrine shells and drift-wood, and,
musing on the changes of which they are the silent yet impressive
witnesses, may sit down to pen a record of the gradual extinction of
the Leman lake on that classic ground where an immortal historian
described the decline and fall of the empire of Rome.
[Footnote 55: Lyell's _Principles of Geology_. Ninth edition, p. 252.]
The alluvial matter deposited by the Rhone at its entrance into the
Lake of Geneva suffers perhaps no change when it once reaches the
bottom. Layer after layer accumulates tranquilly, without disturbance
from surface currents or other causes, so that the renovating effects
of the stream have here every advantage. It is otherwise, however,
where a delta gathers at the mouth of a river upon the sea-margin.
There tides and currents are ever demolishing what the stream has
piled up. Often, too, owing to the prevalence of high winds from
seawards, the river is dammed up for leagues, and the waters of
the ocean encroach far on the delta, mingling in this way marine
remains with those that are fluviatile or terrestrial. But with these
modifications the process of delta-formation remains essentially the
same, both in lakes and at the sea. The vast quantities of sand and
gravel transported by rivers during the flood-season sink to the bottom
as soon as the motion of the water will permit. This takes place at
the shore, where eventually wide tracts of low alluvial land encroach
upon the sea, covered with marshes and overgrown with vegetation. A
section of any of these deltas, obtained in boring for water, shows a
succession of sands and clays, with occasionally a few calcareous beds
and quantities of peaty matter formed of vegetation either drifted
or that grew on the spot.[56] If, now, a sufficient amount of matter
were piled over these loose incoherent strata, they would eventually
become as hard and compact as any of our ordinary building stones. The
sand would subside into a firm compact sandstone; the clay, in like
manner, would consolidate into fissile shale; the peat would become
chemically altered into coal; the calcareous seams would take the form
of layers of limestone; while the leaves, twigs, branches, and trunks,
dispersed through all the beds, would get black and carbonized, so as
precisely to resemble the lepidodendra, calamites, stigmariæ, &c., of
the carboniferous rocks. And thus might a mass of fossiliferous strata,
thousands of feet deep and thousands of square miles in extent, be
amassed by the prolonged operation of a single river.
[Footnote 56: The structure of maritime deltas, especially their
relation to the growth and entombment of forests, will be more fully
alluded to in a subsequent chapter, when we come to inquire into the
origin of a coal-field.]
It often happens that a delta is prevented from extending further
seawards owing to the prevalence of some marine current that comes
sweeping along the coast-line and cuts away the accumulations thrown
down by the river. The sediment thus removed is often carried to great
distances, and eventually settles down as a fine mud along the floor of
the sea, entombing any fucoids, infusoria, shells, corals, fish-bones,
or other relics that may lie at the bottom.
He who has witnessed a storm along a rocky coast-line, has marked the
breakers battering against the weather-bleached cliffs, and heard the
thunder-like rattle of the shingle at the recoil of every wave, needs
not to be told how vast an amount of sediment must in this way be
formed. The pebbles of the beach are ground down still smaller, the
sand produced by their friction finds its way to a lower level, while
the finer particles taken up by the water are borne out to sea, and if
a current traverse the locality may be transported for leagues, till
they at last settle to the bottom. The floor of the sea is consequently
always receiving additions in the form of fine mud--the waste of the
land--derived either from breaker-action, rivers, or icebergs, so
that a series of marine deposits exactly similar to those we find
among the rocks of our hills and valleys, must be constantly in the
course of formation. If circumstances be favourable, the shingle of
the beach may eventually either be covered over or reach a part of
the sea undisturbed by currents or waves, and then consolidate into
what we call conglomerate or pudding-stone. The sand, as before,
becomes sandstone, and the mud laminated shale or hardened clay. These
deposits may go on forming for thousands of years, until at last some
slow elevation or some sudden upheaval of the ocean bed brings them to
the light of day as part of a new continent. Thus exposed they would
differ in no respect from rocks of a similar kind now visible, and
the geologist, in tracing out their origin and history, would have no
hesitation in ranking them among the ordinary marine formations of the
globe.
In fine, we cannot quit the subject without being convinced that
these ceaseless changes afford one of the grandest examples of that
continuous series of mutations--cycle and epicycle--which has been
already alluded to as a distinguishing feature in all the operations
of Nature. We are accustomed to think and speak of "the everlasting
hills." We look on the solid lands whereon we dwell as the emblem of
all that is stable and steadfast, and on the boundless ocean as the
type of all that is unsteady and changeful. The traveller who stands
on those plains where the human race was cradled, marks still the same
valleys with their winding rivers, still the same rocks and hills,
still the same blue sky overhead. The dust of centuries has gathered
over the graves and the dwellings of the early races, yet the covering
is but thin, and if we could conjure from their resting-place some
of these venerable patriarchs, they might perhaps see little or no
change on the haunts of their boyhood. We feel it otherwise, however,
when we contemplate the ocean. In sunshine and in storm its surface
never rests. The wave that now breaks against some bald headland of
our western shores may have come sweeping across from the coast of
America, and the broad swell that rolls into surf along the shores
of Newfoundland may have travelled from the frozen seas of the North
Pole. And so it has ever been; the "far resounding sea" of Homer is the
"far resounding sea" still; and the "countless dimpling of the waves,"
invoked in his agony by the chained Prometheus, remains restless and
playful as ever.
"Firm as a rock," and "fickle as the sea," have therefore become
proverbs of universal acceptance. Yet when we investigate the matter
as we have done in this and the preceding chapter, it appears that an
exactly opposite arrangement would be nearer the truth. It is the sea
that remains constant--
"Time writes no wrinkle on its azure brow;"
while the land undergoes a continual change. Hills are insensibly
mouldering away, valleys are ever being widened and deepened, rocky
coasts and low alluvial shores suffer a constant abrasion, while
even within the bowels of the earth the process of decomposition
uninterruptedly proceeds. And thus, in place of remaining unchanged
from the beginning, we know of nothing more mutable than the land on
which we dwell, so that if the waste everywhere so apparent were to
go on unchecked or unmodified, island and continent would eventually
disappear beneath the waves. Here, however, another principle comes
into operation. The debris removed from the land, as we have seen,
is not annihilated. Slowly borne seawards, it settles down at river
mouths or on the floor of the ocean as an ever-thickening deposit,
which eventually hardens into rock, as solid and enduring as that
whence it was derived. But it does not always remain there. Owing to
the action of subterranean agencies with which we are but slightly
acquainted, different parts of the sea-bottom are continually rising.
Sometimes this process goes on very slowly, as along the shores
of Sweden, where the coast has been ascertained to emerge in some
localities at the rate of about thirty inches in a century; sometimes
with prodigious rapidity, as on the coast of Chili, where the land
was upheaved from two to seven feet in a single night. There can thus
be no doubt that the mysterious agency which produces earthquakes and
volcanoes on the land affects equally that portion of the earth's crust
covered by the waters of the ocean, and must be ceaselessly employed
in elevating large areas of sea-bottom into new continents, that will
ere long become clothed with vegetation and peopled with animals.
In contemplating, therefore, the constant decay in progress on the
surface of the land, we see not a mere isolated process of waste, but a
provision for future renovation. The sandstone cliffs of the shore are
battered down and their debris carried out to sea, but when sea-bottom
comes to be land-surface, they may be sandstone cliffs again, lashed
once more by the breakers, and once more borne as sediment to the
depths of the sea. And thus, in what may seem to us sublime antagonism,
land is ever rising in the domain of ocean, and ocean ever encroaching
on the regions of land. No sooner does a new island, or mountain peak,
or wide area of continent, appear above the waves, than the abrading
agencies are at work again. Rain, air, frost, rivers, currents,
breakers, all begin anew the process of destruction, and cease not
until the land has utterly disappeared, and its worn debris has sunk
in mid-ocean to be in process of time once more dry land, and suffer
another slow process of obliteration.
Such is the economy of nature around us now, and that such will
continue to remain the condition of things in the future, we can affirm
with probability from a consideration of the history of the past. The
geologist can point to masses of rock several miles in thickness, and
occupying a large area of the globe, formed entirely of the worn debris
of pre-existing formations. The very oldest rocks with which he is
acquainted are made up of hardened sediment, pointing to the existence
of some land, even at that early period, worn down by rivers or
wasted by the sea. During all the subsequent ages the same principles
were at work, and now well-nigh the only evidence of the geological
periods is to be gathered from the layers of sediment that successively
settled down at the sea-bottom. The records which it is the task of
the geologist to decipher, are for the most part written in sand and
mud--the deposits of the ocean, for in by far the larger number of
formations into which the stratified part of the earth's crust has been
divided, and which form his only guide to the history of the past, he
can detect no trace of land. Hill and valley have alike disappeared,
and the character of their scenery and inhabitants he can often but
dimly conjecture from the nature of the sediment and of the drifted
terrestrial relics that may chance to be found among strata wholly
marine.
CHAPTER X.
The structure of the stratified part of the earth's
crust conveniently studied by the examination of a
single formation--A coal-field selected for this
purpose--Illustration of the principles necessary to such
an investigation--The antiquities of a country of value in
compiling its pre-historic annals--Geological antiquities
equally valuable and more satisfactorily arranged--Order
of superposition of stratified formations--Each formation
contains its own suite of organic remains--The age of
the boulder defined by this test from fossils--Each
formation as a rule shades into the adjacent ones--Mineral
substances chiefly composing the stratified rocks few
in number--Not of much value in themselves as a test of
age--The Mid-Lothian coal-basin--Its subdivisions--The
limestone of Burdiehouse--Its fossil remains--Its probable
origin--Carboniferous limestone series of Mid-Lothian--Its
relation to that of England--Its organic remains totally
different from those of Burdiehouse--Structure and
scenery of Roman Camp Hill--Its quarries of the mountain
limestone--Fossils of these quarries indicative of an ancient
ocean-bed--Origin of the limestones--Similar formations still
in progress--Coral-reefs and their calcareous silt--Sunset
among the old quarries of Roman Camp Hill.
Among the standard jokes of ancient Athens was that of the simpleton
who, with the intent of selling his house, carried about a brick as a
specimen. In this and the following chapter I propose to follow his
example, and, for the purpose of giving my reader a correct notion of
the structure displayed in the stratified portion of the earth's crust,
to select therefrom a single formation whose details will connect
together the subjects discussed in the previous pages. And in so doing
it will, I trust, be found that what was ludicrous in the hands of
the Greek becomes sober sense in those of the geologist. The "brick,"
then, which I would humbly present to the thoughtful consideration of
the reader as really a specimen of the house of which it forms a part,
has been termed the "Carboniferous System," and consists of a series
of stratified rocks sometimes nearly 15,000 feet thick. The plants
and animals found in these strata have been already described somewhat
in detail, and we have turned aside to look at the processes whereby
such masses of sedimentary rock came to be accumulated. But we shall
probably better understand the habits of the animals and the general
aspect of the vegetation, as well as the agencies at work in depositing
vast beds of mineral matter, if we take a coal-field and analyse it
stratum by stratum, marking as we go their varied and ever-changing
character, and the corresponding diversity of the included organic
remains. Such an examination will bring before us some of the more
striking and important laws of geological research, and while of use
to the young observer, may be not without some share of interest
to the general reader. Before beginning, however, let me endeavour
to illustrate the principles that will guide us by a simple though
hypothetical story.
Suppose the bed of the Firth of Forth were raised above the level of
the sea and covered over with verdure, and that, in ignorance of the
previous topography of the locality, a mason were to excavate on the
lately-born land the foundation for a dwelling-house. Immediately
below the grass he would com? upon layers of hardened mud containing
oyster-beds, with detached valves of cockles, mussels, fish-bones, and
perhaps the tooth of an anchor or the timber of some old herring-boat.
Now, if he were gifted with but ordinary intelligence, what would he at
once conclude from these remains? Plainly, that the spot on which he
stood had once been the bed of the sea. And if in place of appearing
as dry mud and sand these deposits had got hardened into shale and
sandstone, and the shells, too, had become hard and stony, this would
not alter his convictions. He would still assert positively that he
stood upon an old sea-bottom. And suppose further, that all this were
far away from any sea, still such a circumstance could make no change
in his opinion; he would rightly assert that the place of sea and
land might vary, and that the ocean's being now many miles distant
could be no argument against the waves having once rolled over the
site of the intended dwelling-house. Let us further imagine that he
continues his trench, and in sinking deeper comes to a bed of dark
peat with snail-shells and bones of sheep, deer, and oxen. What will
he infer from these? Clearly that they represent an old land-surface,
once covered with vegetation and browsed over by ruminant animals, and
that this old land-surface has at some distant period been submerged
beneath the sea. Suppose, moreover, that below the peat there were a
thin bed of reeds and rushes intermingled with the mouldering remains
of fresh-water shells. He would in that case infer that before the
formation of the peat the locality was occupied by a lake.
Putting now all these deductions together, our mason would have evolved
a very interesting history. He would have ascertained that in a bygone
age the spot on which he stood was the site of a lake, tenanted by
delicate shells and fringed with reeds and rushes, where the coot and
the mallard may have reared their young; that in process of time the
vegetation gained upon the water, choking up the lake, so as gradually
to form a soil firm enough to support sheep, deer, and oxen, and
yielding shady coverts whither the antlered stag could retire and lay
him down to die; that in after years the sea had encroached upon the
peat-moss, and oyster-beds begun to form where cattle had been wont to
browse; that again the ocean receded, and the land emerged to assume
new verdure and receive new inhabitants.
Now, in all this reasoning there is no hypothesis or speculation. The
mason proves himself an intelligent, honest fellow, and uses his eyes
and his head where many other men would perchance see very little need
for the use of either. There can be no setting aside of his story;
he can appeal to facts. "There," says he, "is a layer of peat with
the rush-stalks and moss-fibres matted together in the soft brown
mouldering substance, exactly as I have seen them a hundred times in
the peat-cuttings on the moors, and I cannot but believe that they
must both have had the same origin, that is, that they grew in swampy
hollows of the land. There, too, lies a stratum of fresh-water shells
identical with those that occur in our ponds and marshes. Although
mouldering now, they are evidently not fragmentary, but entire and
unbroken; some of them are young, others full-grown, and they lie
grouped together as in our present lakes. Such shells could only live
in fresh water, therefore the spot where I stand must have been at one
time a fresh-water lake. There, again," he continues, "is a bed of
oysters which cannot have been transported hither, for their valves are
together, lying just as they do in our present oyster-beds. This green
field, therefore, must have been at one period a muddy sea-bottom."
After this manner and upon this kind of evidence must all inquiries
into the past changes of the earth's surface be conducted. And provided
only we proceed cautiously, reasoning from positive facts, and
striving as far as possible to exhaust what Bacon calls the "negative
instances," our deductions possess all the certainty of truth. For in
much the same fashion do we derive no small part of our acquaintance
with the early history of our own land, as well as with the arts
and customs of other nations. The scattered relics turned up by the
operations of the farmer--wooden canoes, flint hatchets, gold torques,
bronze pots, fragments of pottery, and rusty coins--all have their
bearing upon the annals of the country, and so clear is the evidence
which they read out that an eminent antiquary has divided the early
ages of Scotland into three periods, distinguished, from the character
of their relics, as the "Stone Period," the "Bronze Period," and the
"Iron Period."[57] But in such a classification the historian has
little to guide him save the nature of the relics themselves. He places
the rudest first, and groups the rest in succession, according to the
degree of advancement in civilisation which they respectively indicate.
And the grouping seems just, though in some cases objects belonging
to two of these periods may have been to some extent contemporaneous,
just as thatched roofs gave way to tiles, tiles to slates, and slates
partly to lead, though at the present day a walk of half an hour in
some localities will bring before us specimens of all these styles
still in use. If, however, the relics of geological history lay
scattered about like those of early Scottish history, all hope of
ever attaining to anything like a correct chronology and arrangement
would have to be abandoned in despair. In truth, it would then be
impossible to conjecture whether any succession of ages preceded man,
during which other tribes of plants and animals lived and died, or
whether the whole mass of fossiliferous rocks had been accumulated
since the human era, or perhaps created just as we find them. But all
this uncertainty and confusion has been obviated simply by the fossils
being ranged in beds vertically above each other, the oldest at the
bottom and the latest at the top. So that if we find in a low cliff
along the shore blown sand and broken whelks immediately beneath the
vegetable mould, and oyster-valves in a clayey bed three feet below, we
pronounce the oysters to have lived before the whelks, and that between
their respective lifetimes a sufficient interval must have elapsed to
allow three feet of sand, clay, and gravel, to accumulate. What is thus
true on the small scale holds equally so on the large. The stratified
formations in which organic remains occur are found to be grouped
regularly over each other in a settled invariable order. If A be below
B in England it will be below B all over the world, and if C be above
D at the North Pole it will be so at the South Pole too, and at every
locality where the two rocks lie together. This order of superposition
forms one of the grand tests for the age of different rock masses. By
means of this simple rule the geologist has been enabled to arrange
the different stratified formations, supplying the missing portions of
one locality from the more complete series of another, so as to form a
chronological table of no small part of our planet's primeval history.
[Footnote 57: See Dr. Daniel Wilson's deeply interesting work _The
Pre-historic Annals of Scotland_.]
But this is not all. We must attend to the character of the organisms
as well as to their order of occurrence. We must distinguish the animal
from the vegetable, the terrestrial from the marine, and scrupulously
examine the peculiarities of each so as to recognise them again in
other strata. By such careful scrutiny we may trace out the successive
changes in the physical aspect of a district during past times,
viewing in terrestrial plants (when clearly occupying their original
site) evidence of an old land-surface; in _cyprides_, _unios_, and
_paludinæ_, traces of a former lake; and in corals and marine shells,
unmistakable proofs of an ancient sea-bottom. Still further, by marking
the specific character of such fossils we obtain a key to the age of
many rocks that otherwise would be unintelligible, for it is found that
each of the stratified formations, from the oldest upwards, has its own
peculiar and characteristic organisms recognisable all over the world.
This test of the geological position and age of any fossiliferous rock
has a peculiar value, for it can be applied with infallible success
where every other fails. The order of superposition is often obscured
by dislocations and other causes, and the mineralogical texture of a
formation may change entirely in a short space; but if the imbedded
fossils remain, we can be at no loss as to the relationship of the rock
which contains them. And hence, if in some lone island of the Hebrides,
haunted only by the screaming sea-fowl, we find a patch of shale
containing ammonites, belemnites, and a host of other shells in large
measure identical with those occurring among the clays and limestones
of Gloucestershire, we infer that they must all belong to one series
and be of the same age; that, as we know the English beds to form part
of a formation called Lias, of which, the exact place in the geological
scale has been ascertained, so in like manner the Scottish beds must
occupy a position in the same series; and that consequently there was
a time when the site of Cheltenham and part of the Hebrides lay each
beneath a sea which teemed with ammonites, belemnites, and many other
mollusca, along, too, with the bulky saurians of the Lias. And yet
no study of the surrounding rocks in the northern locality, even if
carried on for a thousand years, could ever have thrown one ray of
light upon the subject. In an earlier page our grey rounded boulder
was introduced to the reader as a mass of sandstone belonging to the
Carboniferous group of rocks. How could one be sure of the precise
geological age of a loose water-worn block that might have journeyed
all round the world? Simply by its included fossils. The calamite,
lepidodendron, and stigmaria, revealed the date of the stone as clearly
and unmistakably as if we had seen it lifted from its original bed
by the lever and crane of the quarryman. These plants are peculiarly
characteristic of the Carboniferous strata, and they consequently
stamp as undoubtedly of carboniferous age the rock which contains
them, whether it be sandstone or conglomerate, limestone or shale, and
whether we meet with it among the newly-raised blocks of the quarry,
or among the pebbles of the sea-shore. Each geological formation, I
repeat, beginning at the oldest known to us, and ending with those that
are still forming in our lakes and seas, has its own set of organic
remains whereby we can detect it wherever it may chance to occur, from
the equator to the poles. Each has its _style_, so to speak, just as
we can at once tell whether a drawing represents a Hindoo, Egyptian,
Assyrian, Greek, or Gothic temple, simply from the general _style_ of
the architecture.
Could we but voyage back in time as we can sail forward in space,
we should find each of the geological formations not less clearly
defined than are the different nations and countries of the present
day.[58] Were the reader suddenly set down in an out-of-the-way street
of Paris, he would probably not be long in discovering that he stood
on French ground. Or if spirited away in his sleep he should awake on
the banks of the Nile, he would soon ascertain himself to be in the
land of the Ptolemies. And so if you transported a geologist blindfold
into a quarry where ammonites and belemnites abounded, mingled here
and there with bones of ichthyosaurs and plesiosaurs, he would tell
you at once that the quarry lay among liassic strata. Or if he were
placed in a ravine where the rocks on either hand displayed fern-stems,
lepidodendra, stigmariæ, and sigillariæ, he would tell you that the
surrounding district was one of carboniferous rocks, and that probably
at no great distance there might be found smoking engines and dozens
of coal-pits. Or could you set him down in some dark night upon a
wild coast-line, and show him, perchance by the flare of torch-light,
bones and scales of osteolepis, pterichthys, and dipterus, lying on
the rocks around, he would tell you that the grim crags which shot up
into the gloom were as ancient as the era of the Old Red Sandstone. In
any case the character of the rock would signify nothing, nor would
he care about the general features of the landscape, though these too
become important characteristics in certain cases. Show him but a few
recognisable fossils, and you give him, as it were, an "Open Sesame" to
which the rocks unfold their gates and reveal a store of wonders yet
more varied than those in the cave of Ali Baba.
[Footnote 58: See _ante_, pp. 31, 32, and the Table of Rocks at the end
of the volume.]
But though the geological systems stand thus strongly marked off from
each other when viewed as a whole, their boundary lines can often be
only approximately drawn, thereby reminding us that the divisions are
of man's device, and can have had no place in the plans of Him who
needs not to chronicle His working by years and ages, but with whom
there is no past and no future. One formation insensibly passes into
another just as one nation merges into those around it. There are
sometimes gaps, however, between the formations, serving to mark out
strongly the limits of each,[59] precisely as intervening seas and
mountain-chains serve to mark put the boundaries of different peoples
and tribes.
[Footnote 59: Such cases, however, are probably merely local, and may
have originated from some features in the ancient physical geography
of the districts where they occur. For instance, it has always been
thought that palæozoic ages were marked off by a strong line of
demarcation from succeeding secondary times. But the gap which occurs
in England, France, and Germany, is being slowly filled up from the
evidence furnished by other countries, and we shall probably find in
the end that the Permian dovetailed with the Trias as closely as the
Silurian with the Old Red, or the Lias with the Oolite. In truth, the
longer we study the past history of our planet the less do we see of
hiatus and chasm and sharp clearly defined boundary line; while the
doctrine of a uniform system of laws and arrangements in the physical
world, first philosophically propounded in the immortal "Principles" of
Sir Charles Lyell, is ever receiving fresh confirmation.]
The mineral substances of which these formations consist are
comparatively few in number, being chiefly varieties of sandstone,
shale, conglomerate, and limestone. One sandstone can often be scarcely
distinguished from another, and so also with the other rocks; hence
such tests as mineralogical texture supplies can seldom be relied
on to determine the age of rocks. We can prove, for example, that a
series of limestones in England may be identical in age with a set of
sandstones in Sweden, and with a group of shales in America, because
they all contain the same or representative genera and species of
organic remains. They occupy the same position in the geological scale;
that is, the animals whose fossilized remains lie buried in these rocks
were all living at the same time, while lime was gathering at the
sea-bottom over the site of part of England, and sand was being thrown
down upon a portion of what is now Sweden, and mud was accumulating
over a submerged area of America. In such cases the differences of
mineralogical character go for nothing in determining the age of the
rocks; we have to rely solely on the embedded fossils, and on the order
of superposition.
Keeping in view, then, that the formations into which the geologist
has grouped the stratified portion of the earth's crust have a settled
and invariable order of occurrence, that each of them contains its own
peculiar and characteristic group of organic remains whereby it can
be recognised in any part of the world, and that such remains form
often the sole test at once of the geologic age and of the origin of
the rocks wherein they lie, we may return to the plan above proposed
and endeavour to understand the structure of a coal-field. For this
purpose it may be well to select one of the northern coal-fields of
Britain, since these perhaps display a greater variety in their organic
contents, and bear evidence of more diversified changes in their mode
of formation than can be seen in those of the south. The strata that
compose the coal-basin of Mid-Lothian will probably best suit our
purpose, as they are free from the disturbing effects of those igneous
intrusions which play so important a part among similar rocks to the
north and west.
The Mid-Lothian coal-field comprises a mass of stratified beds of
sandstone, shale, coal, ironstone, and limestone, the united depth of
the whole being above 3000 feet. By reference to the annexed Table it
will be seen that the lowest beds of the section are chiefly sandstones
and shales, extending downwards to an unknown depth, without any coal
that can be profitably worked. These under-strata form the Lower
Carboniferous group. Above them comes a middle zone in which the
characteristic beds are of limestone, comprising the middle portion or
Mountain Limestone of the Scottish Carboniferous rocks. The third and
highest subdivision forms the Upper Carboniferous group or true Coal
Measures, and constitutes the whole of what is properly the Mid-Lothian
coal-field. For the sake of noting some of the remarkable changes
exhibited in the character of the rocks, it may be well to begin our
survey among the upper beds of the under group. Let us take as our base
the famous limestone of Burdiehouse, and work our way upward through
the four thousand feet of strata that lie piled above it.
VERTICAL SECTION OF THE MID-LOTHIAN COAL-FIELD.
UPPER CARBONIFEROUS
OR COAL-MEASURES.
/ / | | A series of sandstones, shales,
| | | | and fire-clays, with interbedded
| =Flat Coal Group.= / | | seams of coal occupying the
| (Above 1000 feet.) \ | | central area of Mid-Lothian
| | | | coal-field.
| \ | |
| |—————————|
| / | |
| | | |
| | | | A great series of sandstones
| | | | and shales with three seams of
/ =Roslyn Sandstone / | | marine limestone (marked here
\ Group.= \ |·········| by dotted lines), With the
| (About 1500 feet.) | | | exception of one or two thin seams
| | |·········| it contains no coal, and serves in
| | | | this way to mark off the coal-
| | | | bearing beds above from the still
| | | | richer coal-bearing beds below.
| \ |·········|
| |—————————|
| / | |
| | | | A group of sandstones and
| =Edge Coal Group.= / | | shales similar to those at the
\ (800-900 feet.) \ | | top, and like them abounding in
| | | coal seams, some of which are
\ | | thick and valuable.
————————————————————————————————————————————————————————————————————————
CARBONIFEROUS LIMESTONES. | |
/ =Roman Camp / | | A set of marine limestones
| Limestones.= | | | intercalated with sandstones,
\ (150-200 feet.) \ | | shales, and a few seams of coal.
————————————————————————————————————————————————————————————————————————
LOWER CARBONIFEROUS. | |
/ | |
| | |
| =Burdiehouse / | | Sandstones and shales extending
| Limestone.= | |·········| to an unknown depth, often
| (27 feet.) \ | | with seams of dull-grey compact
/ | | limestone, rarely of coal. The
\ Thickness of Lower | | beds become very red towards
| Carboniferous Rocks | | the base, and wholly devoid of
| unknown, but | | fossils.
| probably greater | |
| than that of the | |
\ upper. | |
The Burdiehouse limestone is twenty-seven feet thick, of a yellowish
or bluish-grey colour, very compact, splintery, and often fissile
in structure, with a finely striped and laminated appearance, which
probably indicates a slow and tranquil origin. It is Crowded with
fossils, every fragment when taken up showing its seed-cone, fern-stem,
fish-scale, or minute _cyprides_. All the plants seem to belong to
terrestrial species, and have a broken and often a macerated look.
Manifestly they never grew where we now find their remains; they must
have come drifting down from swamp, or jungle, or hill-side. And so
we come to know that during the later ages of the Lower Carboniferous
period, there lay somewhere in the neighbourhood of Burdiehouse a land
clothed with ferns and club-mosses, and through whose swampy hollows
there spread a network of stigmariæ, while sigillariæ waved their
fronds high overhead. From what has been said on a previous page we may
infer that the climate of the old land was moist and equable like that
of New Zealand, nourishing a prolific growth of ferns and other plants
comparatively low in the botanical scale. The scenery of the vegetation
displayed perhaps no great variety of outline, but exhibited rather an
endless succession of the same graceful forms.
But the limestone presents us with other remains than merely those of
terrestrial plants. It displays in abundance the minute dissevered
cases of _cypris_, the small crustaceous animal described above.
Recent species of this genus inhabit stagnant ponds or the bottoms of
gently-flowing rivers, and we hence infer that the ancient species must
in like manner have possessed a similar habitat, and consequently that
the rocks which preserve their remains must have been deposited in
fresh (or, perhaps, brackish) water. Tried by this test the Burdiehouse
limestone must be regarded as a lacustrine, or more probably a
fluviatile formation, which gathered slowly on an undisturbed bottom
swarming with crustaceans and plentifully covered with leaves,
branches, rootlets, and other fragments of terrestrial plants brought
down by streams from the adjoining land. Thus the inferences drawn from
the numerous plants, and from the countless multitude of cypris-cases,
come to be mutually corroborative. The former tell us of some
neighbouring forest-covered country; the latter lead us, as it were,
into its river-mouths, whence we can descry the waving woods on either
side.
Still we have not exhausted all the fossil remains of the Burdiehouse
rocks. Mingled among the stems of ferns and lepidodendra, and the
scattered valves of the cyprides, lie the scales, teeth, and bones,
of several large ganoidal fishes, along with entire specimens of the
smaller genera. The scales of holoptychius are especially abundant,
often crowded together by dozens, and probably not far out of the
arrangement they had when grouped on the body of the living animal.
Detached teeth of the same fish also frequently occur along with
disjointed internal bones. The remains of the contemporary megalichthys
likewise abound, more particularly the scales, which have a fine
nut-brown colour, and dot the surface of the rock with their bright
glittering enamel. Several other smaller ganoids may be met with,
especially a small and elegant species of Palæoniscus (_P. Robisoni_),
and one of Eurynotus, a fish remarkable for the great size of its
dorsal fin. Not uncommon, too, are the ichthyodoralites of a gigantic
placoid--the _Gyracanthus formosus_--with all their delicately-fretted
ornament and a peculiar crystalline glistening surface when broken
across, whereby the smallest fragment can be easily distinguished from
any other bone in the limestone. Such are the ichthyic remains of the
Burdiehouse beds; what deductions can be legitimately drawn from them?
As before, we must have recourse to the analogy of living nature. The
existing ganoidal fishes chiefly inhabit lakes and rivers, especially
near the confluence of the latter with the ocean. They feed on
the decaying matter brought down from the land, or on the minute
Crustacea that swarm upon the river-bottom. If, as seems probable,
the ancient ganoids had habits similar to those of their present
representatives, then the rocks wherein their remains occur abundantly
may have originated on river-bottoms, and such may have been the case
at Burdiehouse. So that here again we have corroborative evidence
of the fluviatile origin of the limestone in question. But besides
the remains of ganoidal fishes there occur the defensive spines of
placoids. Now, the placoids are emphatically marine fishes, and the
sole living representative of the most ancient genera of this order
is the Port-Jackson shark, that haunts the seas round Australia. The
ichthyodorulites of Burdiehouse, therefore, if we would apply analogy
consistently, must be regarded as the relics of marine species. And
this conclusion, too, will be found in entire harmony with those
already obtained, for if we are right in assuming the Burdiehouse
strata to have originated at a river-bottom, particularly near the sea,
we may expect to find the remains of marine predaceous fishes imbedded
in the sediment that gathered there, just as the teeth of the shark may
be preserved among the mud forming in the upper reaches of many British
estuaries, seeing that not a few instances are known where that fish
has been stranded on such shores as those of the higher parts of the
Firth of Forth. These Burdiehouse ichthyodorulites give positive proof
that the limestone could not have originated in a lake, and the only
explanation left is that of a river-bottom.
But it may perhaps be objected that, after all, these fish-remains
are for the most part fragmentary, and may consequently be drifted
specimens, so that no conclusion as to the source of the rock can be
based on their occurrence there. The imbedded land-plants confessedly
came from some distance, why may not the same have been the case with
the bones and scales of the river-haunting ganoid fishes? And, indeed,
did we regard these fish-bones and scales merely in themselves, the
argument might not perhaps be very easily answered, although the great
numbers and perfect outline of the bones, teeth, and scales, afford
pretty strong evidence that the owners lived and died in the locality
where their remains are found. But there is a curious kind of evidence
to be gleaned from the rocks around them whereby this objection can be
at once set aside. In the limestone itself, and especially in some of
the shales above, there occur vast numbers of small oblong coprolitic
concretions of a dirty yellow or brown colour, full of scales and
fragments of bone. There can be no doubt that these are the excremental
remains of predaceous animals, while their great number and perfect
preservation assure us that they could not have been drifted from a
distance, but must rather have been deposited on the spot where we now
find them. And thus we conclude that the site of Burdiehouse must have
been a favourite haunt of these bone-covered fishes; that the bulkier
forms, armed with pointed teeth or barbed-spines, preyed upon their
humbler congeners, while these in turn may have fed on the cyprides
that swarmed by millions at the bottom of the estuary. I have often
detected in these coprolites the peculiarly-sculptured scales of the
palæoniscus. These graceful little animals must, therefore, have died
that their lordlier brethren might dine.
On a survey, then, of the whole evidence from fossils, we are led to
conclude that the Burdiehouse limestone was slowly elaborated at the
bottom of an estuary, into which the remains of terrestrial plants were
drifted from the land, while bone-covered fishes haunted the waters,
and into these busy scenes huge sharks ascended from the sea to share
in the decaying putrescent matter ever brought down from the interior.
The upper part of the limestone is shaly and argillaceous, and rests
below a series of shales and thin sandstones. If the question were
asked, what caused the change from limestone to shale, from the
deposition of a calcareous to that of a muddy sediment, several
answers might be given. The most probable seems to be the following.
The limestone on weathered surfaces displays the mouldering casts of
cypris-cases sometimes in such abundance as to show that the rock must
be largely made up of them. The cyprides of the present day probably
cast their shells annually; the integuments thus thrown off forming
under favourable circumstances a thin mouldering calcareous marl at
the bottom of the pond or marsh, along with the decaying shells of
_paludina_, _planorbis_, _limnea_, or other fresh-water molluscs. We
may conceive the Burdiehouse limestone to have had a similar origin.
The cyprides, inhabiting water that contained little argillaceous
matter, must have propagated by myriads, and during a long period of
repose, in which the conditions of land and sea, and the directions of
tidal currents and river-courses, appear not to have greatly varied
in the neighbourhood of Burdiehouse, the calcareous exuviæ of these
minute animals, along perhaps with the remains of other estuarine or
fluviatile organisms,[60] would form each year a scarce appreciable
stratum, until by slow aggregation a bed twenty-seven feet deep was
elaborated. Each successive annual layer would hardly settle down more
perceptibly or more rapidly than "the flickering dust that mottles the
floor of some old haunted chamber."
[Footnote 60: Though I have never observed molluscan remains in the
limestone of Burdiehouse, they are abundant twelve miles to the west,
in the equivalent strata around Mid-Calder, one little gastropod
being especially plentiful near the base of the calcareous rock in a
seam known to the quarrymen as the "Buckie fake." I have not met with
specimens sufficiently perfect for identification, the hard splintery
nature of the rock seldom allowing anything but a cross-section to be
seen save on weathered specimens, where the general contour of the
shells has sometimes reminded me of _Paludina multiformis_ grouped
together in a recent fresh-water marl. In the shales above the
Burdiehouse limestone, Dr Hibbert states he found a _unio_ (?), called
by him _U. nuciformis_. _Trans. Roy. Soc. Edin._ vol. xiii. p. 245.]
At last, however, this condition of things came to be modified. The
direction of the river channel along some part of its course had
varied, or some analogous change had taken place, so that muddy
sediment transported from the land sank down amid the cyprides at
the bottom. In circumstances so uncongenial these tiny denizens
of the estuary diminished in numbers until the silt and sand came
down so rapidly and in such abundance that they eventually died out.
Alluvial matter still darkened the water and covered the river-bottom,
enveloping now the fronds of a delicate fern that had waved along
the margin of some sequestered lake far inland, anon a seed-cone
that had been shaken by the breeze from the spiky branches of some
tall club-moss. Among these muddy beds occur numerous coprolites and
fish-scales, along with cypriscases and a few shells of unio (?),
showing that though the cyprides were decreasing, the water still
presented the old estuary conditions and still swarmed with life.
Eventually there came other changes in the direction or rapidity of
river currents, and the accumulations of mud and silt were succeeded by
a long protracted deposition of yellow sand, now forming the sandstone
of Straiton. It enclosed many stems of stigmaria, lepidodendron, &c.,
and in certain limited areas these plants matted together in such
quantities that their remains now form thin irregular seams of coal.
It would appear, therefore, that notwithstanding these changes in the
matter transported and deposited at the locality in question, the
estuary character of the locality remained essentially the same. The
sand was at length replaced by fresh accumulations of mud and sandy
silt, which went to form the beds of shale and shaly sandstone now
found above the Straiton rock.
When in the course of many long centuries a depth of strata amounting
to fully 300 feet had been amassed, the area of Mid-Lothian underwent
a total change. Owing to a depression of the earth's crust, that seems
to have been general over the whole of central Scotland, the estuary
in which the Burdiehouse limestone and superincumbent strata were
deposited became open sea. As the evidence of this change rests solely
on the character of the imbedded organic remains, we shall pursue our
induction by examining the beds somewhat in detail.
Rather more than 300 feet above the limestone of Burdiehouse there
occurs in the Mid-Lothian coal-field a series of shales and seams
of limestone. The former are sometimes black and hard, sometimes
bluish-grey, soft, and frequently imbedding the remains of several
genera of mollusca and other organic remains. The limestones vary
considerably in the thickness and general aspect of their several
seams, some being highly crystallized and about two or three feet in
depth, others dull, compact, and ranging up to twenty and thirty feet
thick. The shales and limestones are intercalated with and sometimes
pass into each other, through the gradations of shaly limestone and
calcareous shales. The whole series may measure 150 to 200 feet,
resting on the Straiton sandstone below, and passing upwards into
the under part of the coal-bearing strata of Mid-Lothian known as
the _Edge series_. These limestones form the northern _marine_
equivalents of the mountain limestone of England, while the sandstones
and shales on which they rest, including the Burdiehouse beds and
all the Lower Carboniferous group, must probably be regarded as
_estuarine_ equivalents of the same formation. That is to say, while
marine limestones were accumulating over the site of central England,
sandstone, shale, and drifted plants, were slowly gathering in a wide
estuary over what is now central Scotland, and only at the close of the
period did marine limestones form simultaneously at both localities.
In examining these Mid-Lothian beds we are struck at once with the
great dissimilarity that obtains between their organic remains and
those of the underlying strata. All the land-plants disappear--ferns,
lepidodendra, sigillariæ, and stigmariæ. The cyprides, too, no longer
occur, though the shales seem, at a first glance, to differ in no
respect from those underneath, in some of which the cypris-cases were
seen to abound. Neither can we detect the glittering scales and teeth
that stood out in such strong relief upon the rocks below. Yet the
fossils are scarcely less numerous than they were in the lower beds.
Nay, in some of the limestones they lie so crowded together that the
rock seems entirely made up of them. Plainly such a total renovation
of organic life points to some equally extensive change of a physical
kind. Let us examine for a little some of the fossil remains occurring
in the mountain limestone series of Mid-Lothian, and read off, if we
can, the revolutions which they chronicle.
[Illustration: Fig. 32.--Section from Gilmerton to Crichton; _a_, Lower
Carboniferous; _b_, Mountain Limestone; _c_, Edge Series; _d_, Roslyn
Sandstone Group; _e_, Flat Coals; _y_, Drift.]
The neighbourhood of Edinburgh affords many facilities for the study of
these rocks. They can be seen, for instance, at many points along the
ridge of the Roman Camp Hill, near Dalkeith, exposed in the operations
of quarrying. That ridge is formed by what is known technically as
an anticlinal axis (Fig. 32); in other words, the lower beds of the
coal-measures rise up here into a sort of broad wave-like undulation,
round the sides of which the higher parts of the series are folded. The
elevated area has either been pushed up from below, or the more level
country around has subsided into two trough-like hollows, so that now
the strata, which geologically speaking are lowest, come to occupy the
highest ground in the district. Seated on some of the opener spots
of this woody eminence the observer has a noble prospect on which to
expatiate. The ground around him is rich in historic associations,
and links itself to many a varied page in the annals of Scotland. The
hill on which he rests is crowned by the mouldering mounds of what
tradition reports to have been a Roman station, but which may perhaps
belong to a still earlier era. A few hundred yards north rise the
wooded slopes of Carberry Hill, where the hapless Mary surrendered to
her rebel lords, and whence she was led into her own capital amid the
insults of an infuriate rabble. Northward, too, lies the fatal field
of Pinkie, and eastward the less deadly but not less decisive field
of Prestonpans. To the west the eye can mark the grey smoke of the
Scottish metropolis, with its dusky towers and its lion-shaped hill,
and then the blue waving outline of the Pentlands that sweep away
south and lose themselves among the distant hills which girdle in the
coal-basin of Edinburgh and Haddington. The course of the Esk--that
_fabulosus amnis_--passes by many a time-honoured spot, from Habbie's
Howe and the scene of the Gentle Shepherd down by the haunted scenery
of Roslyn, the cliffs of Hawthornden, the grounds of Newbattle, and
the old Roman station of Inveresk. East, west, and south, the broad
expanse of green field and clustering wood swells upward to the distant
hills that encircle the landscape with a wavy line of softest blue.
Northward the eye rests on the Firth of Forth with its solitary sails,
bounded by the bosky heights of Fife, and opening outwards by the May
Island and the Bass Rock into the far-off hazy ocean. On every side
objects of historic interest lie crowded together, about which many
pleasant volumes have been and might still be written. If the observer
be a lover of geological science he will find an examination of the
structure of the hill to impart an additional interest to the scene.
From the wide panorama of hill and dale, river and sea, with all its
battle-fields, castles, and abbeys, and all its memories of the olden
time, let him turn into one of the quarries that indent the flanks
of the hill, and try to decipher there the records of a still older
history. An hour or two thus spent will pass swiftly and pleasantly
away, and on quitting the quarry he will have gained a new light in
which to look on the landscape that lies spread out below.
The mountain limestone of Mid-Lothian consists, as has been mentioned,
of several seams interbedded with black and calcareous shales. The
quarries on Roman Camp Hill have been opened in several of the thickest
of these seams. Let us enter one of the excavations. A vertical face
of rock forms the background, overhung above by long dangling tufts of
withered grass, and washed below by a pool of water having that milky
green tint peculiar to old lime-quarries. The lowest rock visible is
a dull grey limestone with a yellowish weathered surface. Above it
rests a mass of hard yellow calcareous shale, known to the workmen
as "bands." This rock is worthless as a source of lime, nor from its
irregular laminations and shivery structure has it much value in any
other way. A few inches of surface-soil form the upper part of the
section. It requires but a glance over the weathered surface of the
limestone to mark that the rock abounds in fossils. Of these by far
the most numerous are the joints of the stone-lily, for the most part
of small size, and when broken across, with their minute central
apertures, looking like so many fractured stems of tobacco-pipes. Other
organisms also occur, such as a small delicately-plaited productus,
a larger and more boldly-ribbed spirifer, a small cyathophyllum or
cup-coral, and the fragile interlacing meshes of one of the net-like
bryozoa--the fenestella. Of rarer occurrence are the whorled shells
called bellerophon, the long chambered shells of orthoceratites, and
the grooved tapering shells of pinnæ. Many of the same fossils can be
detected in the beds above, which thus evidently all form part of one
series with the rock below. What, then, were the circumstances under
which these strata originated?
The answer to such a question is not far to seek. The corals and
crinoids are exclusively marine families, and so any stratum in which
their remains occur must have had a submarine origin. It matters not
in this case though the specimens be fragmentary, showing a broken and
drifted appearance. For even supposing that they did not live at the
spot where their petrified relics are now exhumed by the operations of
the quarryman, granting that they were drifted from a distance, still
they could only have been drifted from one part of the sea-bottom to
another. The state of keeping of the specimen often tells vastly on the
value of its evidence when it belongs to a land or fresh-water tribe.
Thus, in one of the limestones of West-Lothian I have found a black
carbonized stem of sigillaria. Now, the sigillaria was a land-plant
as much as any of our hazels or willows, and where the evidence from
the associated organisms coincides, furnishes its own testimony as
to the origin of the rock which imbeds its remains. But the stem in
question was a mere fragment, and showed moreover a worn macerated
surface. Such a fossil had evidently no value as a test of the origin
of the limestone, which might have been elaborated either in an inland
lake or in open sea. That it had really a marine origin, and that the
sigillaria actually was, as it seemed to have been, a drifted plant,
I ascertained beyond a doubt by detecting on the same slab hundreds
of encrinal stems along with the shells, and thin, delicate, silvery
spines of productus. Thus, then, the organisms of the land may be
carried into the sea, and in dealing with their fossilized remains
in the deposits of former ages we must be very careful in the use of
evidence derived from fragmentary and drifted specimens. But no such
caution is needed in regard to the productions of the sea. If they be
fragmentary and drifted, we may believe they were rolled about by tides
and currents previous to their final entombment; but still they remain
as good a test as ever of the marine character of the rock in which
they occur.[61]
[Footnote 61: The exceptional instance, of the accumulation on the land
of blown sand imbedding the broken remains of marine shells, needs only
to be noticed here.]
The fossils of Roman Camp Hill are not drifted specimens. They must
have lived and died where the quarryman now finds them. We recognise
them as all unequivocally marine; corals, crinoids, and brachiopodous
molluscs, are all clearly the denizens of the sea, and hence we
conclude that they mark the site of an ancient ocean. The snail-shells
that swarm about the fruit-trees of our orchards not more unmistakably
indicate a land-surface than do these petrified relics evidence an old
sea-bottom. We can argue, too, from the crowded way in which they lie
grouped together, that life must have been prolific in these primeval
waters. Every fragment of the rock shows its dozens, nay, hundreds,
of stone-lily joints, disjointed indeed, yet easily recognisable.
They must have swarmed as thickly along the floor of the sea as the
strong-stemmed tangle that darkens the bottom of many a picturesque
bay along our western coasts, yet with a gracefulness of outline such
as none of our larger sea-weeds can boast. Less numerous but not less
markedly _in situ_ are the shells of productus and spirifer, the former
with its finely-striated surface fresh as if the creature had died but
yesterday, while the slender spines with which it was armed lie strewed
around. In short, the whole suite of organisms points to a period of
tranquil deposition in a sea of probably no great depth, where the
lower forms of the animal kingdom flourished in abundance, contributing
by their calcareous secretions to form continuous layers of limestone.
Such a condition of things finds a parallel in many parts of the globe
at the present day. Thus, the shores of the islands of the Pacific
are white with fine calcareous mud, that results from the action
of breakers on the surrounding coral-reefs. This mud, enveloping
fragments of coral, shells, sea-weed, drift-wood, and other extraneous
substances, hardens on exposure, and becomes eventually a limestone,
travertine, or calc-sinter. We may believe that the same process goes
on out at sea, around the edges of atolls or circular coral-reefs,
and that the sediment thus thrown down will enclose any zoophytes
or molluscan remains that may lie at the sea-bottom, along perhaps
with _fuci_, and occasional water-logged fragments of wood that have
been drifted from land. Along the shores of Guadaloupe a bed of this
calcareous silt has formed since America was colonized by man, for it
has been found to contain fragments of pottery, arrow-heads, and other
articles of human workmanship.[62] The same rock has yielded, besides,
the partially-petrified bones of several human skeletons, one of which,
though without the head, forms a prominent object among the fossil
treasures of the British Museum. The rock in which these remains are
embedded is described as harder than statuary marble, notwithstanding
its recent origin. By supposing the same process to be carried on over
a large area and for a long period, we may see how a continuous stratum
of limestone could be elaborated, full of fossil relics of corals,
molluscs, and other marine productions. And in some such way, we may
be permitted to believe, the seams of limestone on Roman Camp Hill
were accumulated. The billows of that old carboniferous ocean may not
have sent up their white surf against the margin of snowy coral-reefs,
but the currents below did their work of demolition as effectually,
and by sweeping through the submarine groves of stone-lilies and
cup-corals, as the night winds of autumn sweep athwart the heavy-laden
fields, would prostrate many a full-grown stem and scatter its
loosened joints among the thickening lime that covered the bottom.
Stone-lily, cup-coral, net-coral, productus, spirifer, pinna, nautilus,
orthoceratite, all would eventually be entombed amid the decaying
remains of their congeners, and thus produce a slowly-increasing seam
of limestone.
[Footnote 62: Lyell's _Manual of Elementary Geology_, p. 121. Fifth
edition.]
We still linger in the old quarry on Roman Camp Hill, but the day
draws rapidly to a close, and the long level beams of the setting sun
lighten up the higher grounds with a golden flush, while the valley
below lies deep in shade. The rays fall brightly on the abrupt face of
limestone at the further end of the quarry, every prominence standing
out in bold relief, and casting its shadow far behind. Our eye, in
passing over the sunlit rock, can detect the fractured joint of many an
encrinite glancing in the light; along, too, with the strongly defined
outlines of some of the lesser and more abundant molluscs--spirifers
or producti. Some of them, sorely effaced by the rains, have begun
to yield a scanty nestling place for creeping fibres of moss; others
yet bare, afford a rest to the _Vanessa_ whereon to spread its wings
in the mellow sunset ere flitting homewards among the dewy herbage.
The bushes overhead scarcely rustle in the light-breathing air that
comes fitfully across the land, and the long grass nods dreamily on
the margin of the pool below. There rests a calm stillness on all the
nearer landscape, and the distant ground blends away into the shades
of evening. The scene, in short, has about it that solemn impressive
repose which irresistibly arrests the fancy, and sets it to dress up
into fantastic shapes the massive clouds that float in the western
sky, to picture grim forms amid the misty shadows of the valley, or to
dwell half dreaming upon the memories of the past, that come crowding
through the mind in quick succession. Our labours among the fossils of
the old quarry, however, enable fancy to draw her stores from another
source. We muse on these petrified relics, gilded by the last rays
of the setting sun, when slowly, like a dissolving view, sunset and
herbage melt away, and the bottom of the old carboniferous ocean lies
before us with its corals and shells and stone-lilies, stretching out
their quivering arms, or expanding and contracting their flower-like
petals amid a scene of ceaseless animation and activity. Geology
delights in contrasts, and assuredly the contrast presented to us this
evening between the present and the past of Roman Camp Hill, will not
rank among the least striking of those which she has to reveal. There
is now spread over us the blue sky, richly hung with tinted clouds,
and melodious with the evening songs of the lark, the blackbird, and
the thrush. Not less surely did a wide expanse of sea during the
Carboniferous era roll over the hill on which we stand. And yonder
silvery moon that mounts up amid the violet twilight of the east, has
witnessed each scene and all the countless changes that have intervened
between them. The same pale light that now begins to steal through the
woods and athwart the fields, must have streamed down upon that old
sea and illumined its green depths. Oceans and continents, islands and
lakes, hills and valleys, have come and gone with all their successive
races of living things, and that same planet has marked them all. She
has seen, too, as but a thing of yesterday, the appearance of man upon
the scene, with all the successive centuries that have elapsed since
then. Truly the "goddess of the silver bow" would have a strange story
to tell us could we interrogate her about the past. But the days of
Endymion have gone by, and she now no longer visits in a personal form
the seat of beings who gaze at her crescent orb and daringly pronounce
it a scene of blasted ruin and desolation.
CHAPTER XI.
Intercalation of coal seams among mountain limestone beds of
Mid-Loihian--North Greens seam--Most of our coal seams
indicate former land-surfaces--Origin of coal a debated
question--Erect fossil trees in coal-measures--Deductions to
be drawn therefrom--Difference between the mountain limestone
of Scotland and that of England--Coal-bearing character of the
northern series--Divisions of the Mid-Lothian coal-field--The
Edge coals--Their origin illustrated by the growth of modern
deltas--Delta of the Nile--Of the Mississippi--Of the
Ganges--Progress of formation of the Edge coals--Scenery
of the period like that of modern deltas--Calculations of
the time required for the growth of a coal-field--Why of
doubtful value--Roslyn Sandstone group--Affords proofs of
a general and more rapid subsidence beneath the sea--Its
great continuity--Probable origin--Flat coals--Similar
in origin to the Edge coals below--Their series not now
complete--Recapitulation of the general changes indicated by
the Mid-Lothian coal-field.
Among the old quarries of Roman Camp Hill and down the course of
several streams in the same county, the limestone beds of the mountain
limestone series are seen to be associated with strata of shale, some
of which are highly calcareous, and charged with the same organic
remains that occur in the limestones. Such shaly intercalations mark
as before the transport and deposition of muddy sediment around and
above the corals and stone-lilies of the sea-bottom. All these beds
must undoubtedly be regarded as marine. But there occur, besides,
seams of sandstone and black partially-bituminous shale, with layers
of coal and fire-clay. To this singular intermixture it may be well
to advert more particularly, since it forms one of the distinguishing
features of these northern rocks, as contrasted with those of central
and south-western England, and more especially since it will lead us
to mark again the value of fossil remains as evidence of the ancient
changes of land and sea.
The southern part of Mid-Lothian consists of a broad heathy moorland,
that slopes northward into the more cultivated country, and swells
upward to the south into the series of undulating ridges that form the
Moorfoot Hills. It is traversed by several streams which rise high
among the pasture grounds of the south, and flow some into the valley
of the Esk, and thence into the sea at Musselburgh; others past the
ancient fortalices of Borthwick and Crichton, and so by the valley of
the Tyne into the sea at Tyningham. In their upper course they traverse
a broad belt of the mountain limestone that stretches across this
part of the country from east to west, and dips away north under the
coal-field. Where the streams have been able to cut through the thick
mantle of heath, sand, gravel, and clay, by which these higher grounds
are covered, we sometimes obtain a complete section of the strata
displayed in regular sequence along the bottom of the channels. Thus,
one of the rivulets that trickles slowly through the swampy ground of
Middleton Muir, on approaching the line of limestone begins to descend
more rapidly, and has excavated its course through several feet of the
rock below. The limestones are well exposed along each side of the
stream, forming in some places steep walls tapestried with moss and
overhung with scraggy furze, and offering to the student an instructive
series of sections. Near the farm of Esperston, where the stream flows
through a narrow secluded valley, the limestones form a floor which the
water in the course of centuries has worn smooth, so that the rock with
its included encrinal stems and shells, polished by the ceaseless flow
of the current, shows like a sheet of variegated marble. At one point
on the side of the water-course the observer may notice a low ledge
of rock jutting out for a short way along the edge of the stream. The
upper part is a hard compact limestone, full of small crinoidal joints.
The bed underneath it has been greatly eroded by the rivulet, but
enough remains to show that the stratum is one of coal. It rests upon
the series of limestones and sandstones seen in the upper part of the
water-course, and is surmounted by the thick limestones of Arniston and
Middleton. A similar seam nineteen inches thick has been worked among
the limestone about three miles to the west at Fountain. The same bed
occurs among the quarries on Roman Camp Hill already mentioned, and I
have seen an equivalent stratum intercalated among sheets of cup-corals
and stone-lilies on the shore at Aberlady, where the waves have laid
open perhaps the finest section of Carboniferous limestone strata in
Scotland. In West-Lothian, too, the same intercalation of coal-seams
among the mountain limestone beds can be seen in many places. Thus, in
the bed of the River Almond, near Blackburn, the following section is
laid bare:--
Calcareous shale.
Limestone (marine), eight feet.
Calcareous shale, with _spirifers_, &c.
Coal, six to eight inches.
Fire-clay.
Sandstone.
A short way further down the stream another bed of limestone occurs
with several seams of coal below it, one of them attaining a thickness
of six feet.
In addition to the thin seam at Esperston, the Mid-Lothian field
contains several others. Of these by much the most important is that
known as the North Greens Seam. It varies in thickness from only a
few inches to fully 5 feet, and has been extensively worked for the
_parrot_ or gas-coal which it contains. It rests upon a pavement of
shale, sometimes of fire-clay, and occurs about midway between two
thick marine limestones, being from 80 to 90 feet distant from each.
I have laid open many a block of the parrot-coal at the pit mouth,
and marked the well-defined outlines of the stigmaria covered with a
yellowish efflorescence of iron pyrites, like gilded figures upon
a black velvet ground. The plants lie with their divergent rootlets
spread out regularly along the stem like teeth on the back of a comb,
thus seeming to indicate no hurried agglomeration by some tidal wave or
turbid river, but rather a slow and tranquil deposition.
The fossils of the coal-seams consist for the most part of the
plants above described, which we saw to belong to _terrestrial
species_. But the reader will now understand that in dealing with
organic remains we cannot infer, because a certain stratum contains
nothing but land-plants, that it must necessarily by consequence be a
land-formation. For we have seen that the plants of the Burdiehouse
limestone, though all terrestrial, gave no support to the idea that the
rock had originated on land. In all such cases regard must be had not
only to the nature of the imbedded organisms, but their condition and
mode of occurrence, and to the character of those associated with them.
Especial care must be taken to distinguish what has been transported
from what is _in situ_, otherwise, by attending only to one part of
the evidence, we shall miss the import of the whole, and altogether
misinterpret the records which we seek to decipher.
For years the subject of the origin of coal formed one of the many
battle-fields on which geologists delighted to break lances. They
ranged themselves under two banners, the "drift"-theory men and the
"growth"-theory men, the former maintaining strenuously that coal
was simply vegetation transported from the land and deposited in
large troughs at river-mouths or sea-bottoms, the latter as eagerly
contending that the vegetation had not been drifted, but grew on
the very locality where its remains are now exhumed. Neither party
lacked plausible arguments in support of its doctrines. The "drift"
combatants stoutly affirmed it to be contrary to all experience that
a land-surface should be so oscillating as their opponents required,
that in short it was absurd to hold each coal-seam as marking a period
of elevation, for there were often dozens of seams in as many yards
of strata, some of them scarcely an inch thick, and yet, according to
the "growth" theory, each would have required for its accumulation
a special uplifting of the land above the sea-level. These and many
other difficulties were thought to be triumphantly overcome by the
hypothesis of transport and deposition. The vegetation borne down
by some ancient Mississippi would collect in vast rafts, and these
becoming water-logged would sink to the bottom, where, by getting
eventually covered over with silt and sand, they would in process of
time be chemically altered into coal. This explanation was, however,
vigorously resisted by the opposite side. They alleged that the "drift"
theory could account neither for the wide extent of coal-seams nor
for their remarkable persistency in thickness. If the vegetation had
really been hurried out to sea by river-action, it seemed natural to
expect that the coal-seams should occur in sporadic patches of very
unequal thicknesses, according as the drifted plants had been more
densely or more loosely packed. But this was found not to be the case
in point of fact. The coal-seams were ascertained to be generally
singularly continuous, and to retain for the most part a pretty uniform
thickness over considerable areas. And what was still more worthy of
note, they were, as a whole, markedly free from extraneous matter,
such as sand and mud. Where these impurities did occur, it was usually
in the form of intercalated seams or partings, often quite as regular
and extensive as the coal itself. Had the vegetation, therefore, been
transported into the sea, it could hardly fail to get mixed up with
the fine impalpable mud which, like that of the Ganges or Mississippi,
might have discoloured the ocean for leagues from the river-mouth,
and settled down as a thickening stratum at the sea-bottom. And many
other arguments, derived from the nature and arrangement of the strata
interbedded among the coal-seams, were urged to prove that the latter
had originated from vegetation which grew on the spot.
[Illustration: Fig. 33.--Section from Cape Breton coal-field, showing
four planes of vertical stems, and seven ancient soils with their
covering of vegetation.
_a_, sandstones; _b_, shales; _c_, coal; _d_, fire-clays; _e_,
arenaceous shales.]
The warfare seems now pretty nearly at an end, and as often happens in
such cases, it has been found that each party was to some extent in the
right and to some extent in the wrong. It has been ascertained that
some coal-seams must have originated from the deposition of drift-wood
in the mud and ooze of the sea-bottom, while others undoubtedly arose
from the decay and entombment of vegetation in swampy plains of the
land. That the latter mode of formation has been the usual one in most
of our coal-fields has been generally acknowledged since Sir William
Logan's announcement that each coal-seam, for the most part, rests
upon a bed of fire-clay, which, with its embedded roots, marks the
site of an ancient soil. This fact has been abundantly confirmed in
every part of this country, and indeed wherever an extended series of
coal-seams has been examined. Not only have the underlying fire-clays
been found, but in not a few instances erect stems of trees, passing
down through the coal-seam and spreading out their divergent roots in
the clay below, exactly as they must have done when they flourished
green and luxuriant in the times of the Carboniferous system. This was
especially the case in the Parkfield Colliery, Wolverhampton, where
seventy-three trunks were laid bare in the space of about a quarter of
an acre, each with its roots attached. The same appearance was observed
some years ago in the Dalkeith coal-field, where a group of erect
trees was encountered covering a space of several square yards. Some
instructive sections of such fossil-forests are given by Mr. Brown from
the Cape Breton coal-field.[63] In one of them (Fig. 33) no fewer than
four planes occur, each supporting its group of erect steins. Now, no
one can glance over this and the other sections illustrative of the
same paper, or the descriptions given by Sir Charles Lyell and others
of the Nova Scotian coal-field, without being compelled to admit that
the trees in question grew just where their upright stems can still be
seen, and consequently that the accompanying coal-seams originated not
from vegetation drifted by river-action, but from vegetation that grew
upon the spot. And though erect stems do not exist in every coal-field,
we seldom fail to detect the not less important occurrence of the
fire-clays and hardened shales that support the coal-seams and prove
by their embedded rootlets their identity with ancient soils. Thus we
arrive at the inference that while in certain localities coal-seams
have resulted from drifted vegetable matter, they have nevertheless for
the most part been formed from plants that flourished where the collier
now excavates, amid damp and dripping caverns, their carbonized remains.
[Footnote 63: _Quart. Jour. Geol. Soc._ vol. vi. pp. 120, 130. The cut
given above (Fig. 33) is taken from one of these sections as modified
by the late Sir Henry de la Beche (_Geological Observer_, p. 582). In
the original the beds are inclined at a considerable angle, but for the
sake of clearness they are here reduced to horizontality.]
Applying, then, this deduction to the strata occurring on the horizon
of the mountain limestone in Mid-Lothian, we are led to believe that
the North Greens coal-seam marks the site of a former land-surface.
It shows no vertical stems, but has all the other accompaniments of
an ordinary seam, such as the underlying fire-clay and shale, with
their included stigmariæ. And this conclusion has more than ordinary
interest, for if it be true, we have evidence of a terrestrial
formation among strata unequivocally marine; in other words, we see
proofs either of an elevation or a filling-up[64] of the sea-bottom
carried slowly on until land-plants grew up in matted swamps where
once there swarmed corals and encrinites, and then of a gradual
subsidence, so that marine organisms flourished again in abundance
over the site of the submerged vegetation. It is not insisted that
each of the thin coal-seams among the limestone strata marks a former
terrestrial area. Some of them may possibly have resulted from the
transport and deposition of plants borne from the land. Yet there are
others of wide extent resting upon beds of fire-clay which contains
stigmaria rootlets, &c. These I cannot but regard as the remains of
plants that grew upon the spot. And so, while we recognise in the beds
of limestone undoubted evidence of a former sea-bottom, I am persuaded
we must equally admit that at least several of the coal-seams bear fair
evidence of a former land-surface, scarcely raised above the sea-level
indeed, but nourishing nevertheless a thickly matted vegetation. In
this way we shall see the mountain limestone series of the Lothians
to be not a purely marine formation, but one partly marine and partly
deltoid, showing in the succession of its strata proofs of a gradual
submergence, interrupted by movements of elevation, so that the area
which at one period formed the ocean-bed became at a later time low
delta-land, and after continuing perhaps for ages to stretch out its
verdant surface beneath the open sky, sank again amid the corals of a
wide-spread sea.
[Footnote 64: If it be correct to set down the North Greens coal-seam
as really representing a terrestrial surface, that is, of course, a
flat delta or plain scarcely raised above the sea-level, we must, I
suspect, call in the aid of a slight elevatory process, or else hold
that the depth of the sea at the locality where the lower limestone was
forming did not exceed 80 or 90 feet, and may have been considerably
less, and that this space came to be eventually filled up by the
detritus of the river. But the wide extent and sometimes the great
thickness of the limestone beds seem to indicate a greater depth, and
thus favour the idea of an elevation of the sea-bottom to form the
North Greens coal-seam.]
Now this condition of things differs entirely from what is presented
by the Mountain Limestone group of England. That formation, when
typically developed, attains a thickness of from 1000 to 2000 feet, and
gives rise to that green hilly kind of scenery whence it has derived
its name. It is unequivocally a marine deposit, since it abounds in
corals, echinoderms, brachiopodous molluscs, and other productions of
the deep. Northward, however, it undergoes a gradual change, getting
greatly thinner, and split up by a series of intercalations of shale
and sandstone. This alteration goes on until, on the border-land
between the two countries, the massive limestone of Derbyshire has
dwindled down into a series of thin beds, often widely separated by
intervening strata, which contain many seams of coal. After crossing
the Silurian district, and descending the northern slopes of the
Lammermuir Hills, we get into the Carboniferous system again, and find
its limestone series still farther diminished. With this decrease
of marine formations, we can detect an augmentation of coal-bearing
strata. Thus the Berwickshire coal-field lies in this lower set of
beds, far under the coal-measures of Newcastle. In the Lothians, too,
as has been shown, coal is extensively worked in the same series, and
these seams also find their representatives in Fife and Lanarkshire.
The gradual change from the kind of strata found on the horizon of
the Burdiehouse limestone, to those occurring on the horizon of the
Mountain limestone, indicates, as we saw, a gradual change of the
conditions of deposition; and the nature of this alteration is shown by
the difference in the character of the imbedded fossils. The passage
of the massive Derbyshire limestone into the thin limestones and
coal-bearing sandstones of the north, as decidedly marks another change
in the relative position of sea and land. The former was a succession
in time, the latter was one in space, but the mode of reasoning remains
the same for both. In the former case, we saw estuarine strata passing
upward into others wholly marine, and the order of superposition told
us that the locality was first an estuary, and then slowly became open
sea. In the latter case, we see marine beds not succeeded by estuarine
strata, but becoming estuarine strata themselves. The thick limestones
gradually thin out horizontally into a great series of sandstones and
shales, with interbedded coal-seams, so that what bears evidence of a
deep sea at the one end, gives proof of a muddy and sandy delta at the
other. In other words, during the ages represented by what we call the
Mountain Limestone, the central and south-western portions of England
lay far below a wide breadth of ocean, and nourished a luxuriant crop
of stone-lilies, mingled with the other denizens of the deep, while the
Border district, and the whole of central Scotland, exhibited all the
conditions of a vast delta, sometimes spreading out as broad verdant
jungles, anon showing only scattered irregular groups of low, bare
mud-banks and sand-spits, which at other times disappeared altogether
beneath the dun discoloured waves. Now the reader will not fail to
mark that this curious and interesting fact in the past history of our
country, is ascertained solely from a comparison of fossil remains.
The stone-lilies and shells of Derbyshire, and the lepidodendra and
land-plants of the Lothians, form our sole basis of evidence, and we
may rest on them with as perfect certainty as if they were so many duly
attested documents deposited among the archives of our State-Paper
Office.
In our survey of the coal-field of Mid-Lothian, we have passed from
the Lower Carboniferous estuary beds of Burdiehouse to the Middle
Carboniferous marine beds of Roman Camp Hill, and their associated
terrestrial strata,--the coal-seams and fire-clays. We come now, in our
upward progress, to the Upper Carboniferous group, or Coal-measures
proper.[65] These strata rest immediately upon the limestones, and
attain a depth here of over three thousand feet. They consist of a
great series of sandstones, shales, coals, and fire-clays, that vary
in thickness from less than an inch to many feet, or even yards. The
coal-seams are especially variable, many of them existing as mere films
of carbonaceous matter; others varying up to a depth of fourteen feet.
There are from fifty to sixty that exceed a foot, but the average
thickness throughout the whole series is about three and a half
feet.[66] They are nearly all underlaid by fire-clay or shale, and very
generally have a roof of the latter material.
[Footnote 65: These terms--Lower, Middle, and Upper Carboniferous, are
used for want of others, and for the sake of clearness. They must not
be regarded, however, as equivalent to similar groupings of the English
carboniferous rocks, for the Scottish series is probably much older
than the greater part of the English, and coeval, to a considerable
extent, with the mountain limestone and millstone grit of the latter
country.]
[Footnote 66: See Milne on Mid-Lothian Coal-field. _Trans. Royal Soc.
Edin._ vol. xiv. p. 256, whence the above details are taken.]
By referring to the diagram of this coal-field, given above at p.
196, the reader will notice that the series is divisible into three
groups:--1_st_, and undermost, a considerable depth of coal bearing
strata known as the _edge series_, because they lie along the western
limits of the coal-basin at a high angle, and sometimes even on
edge; 2_d_, A great thickness of sandstones nearly barren of coal,
but containing at least three beds of limestone this may be termed
the Roslyn sandstone group; 3_d_, and highest, another series of
coal-bearing strata, commonly called the _flat coals_, because they
occupy the centre of the basin where the beds repose at a low angle,
and are in places quite flat. It will be convenient to keep in mind
this three-fold division, for it will point us to some important
changes in the ancient conditions of this coal-field.
The edge series, which forms the lowest, and of course oldest of the
above groups, averages from 800 to 900 feet in thickness. It contains
about thirty seams of coal above a foot thick, and many more of less
size. They occur irregularly, some lying only a few inches apart,
others from eighty to ninety feet, the intervening space being occupied
by sandstone or shale.
Now as each coal-seam, with its associated under-clay, appears to mark
a former land surface, it will follow that there must be as many old
land surfaces in this series of strata as there are such coal-seams,
and that for every intervening mass of sandstone or shale, the area
of vegetation must have been submerged. This conclusion would have
been violently resisted by the supporters of the "drift" theory. They
would have roundly asserted that such an unsteady surface was a mere
supposition to suit a hypothesis, unsupported by fact, and contrary
to the analogy of existing nature; and they would not perhaps have
hesitated to maintain, that such an oscillating land could be little
fitted to nourish so rich and luxuriant a vegetation as that of the
Carboniferous period. But it will not be difficult to show that our
conclusion, so far from being contrary to analogy, is amply borne
out by the processes of existing nature, and that its opponents, and
even its original asserters, failed to perceive that what it demands
is not a rapidly oscillating crust, but one as steady and uniform as
that of many of the least disturbed countries at the present day;
and that we do not require to call in the aid of a special elevation
and submergence for every coal-seam, but that for the most part the
hypothesis of a steady sinking of the area of a coal-field, interrupted
perhaps by occasional elevatory movements, along with an active and
constant deposition of sediment by the varying currents of a large
river, is sufficient, if not thoroughly to explain, at least to
throw great light upon the origin of those enormous masses of strata
composing our present coal-basins. The oft-recurring variations in
the nature of the strata that form our coal-measures, sandstones
alternating with shales, these again with coals and fire-clays,
together also with the terrestrial origin of the coal-seams, and the
occasional presence of true marine organisms, make it evident that,
to obtain any modern analogue to such a condition of things, we must
examine those localities where large bodies of fresh water, carrying
sediment and vegetation from the land, mingle with the sea. Let us then
look for a little at the operations now in progress at the mouths of
the larger rivers, and mark how far they elucidate the structure and
history of a coal-field.
"Egypt is the gift of the Nile." Such was the conclusion arrived at by
one of the most diligent observers of ancient Greece--the venerable
Herodotus.[67] He sailed up the river marking all the leading features
in its scenery, and noting the more apparent evidences of ancient
physical changes. His remarks on these subjects form one of the
earliest specimens of scientific reasoning that have come down to us,
and are remarkable for their correctness and the truly inductive mode
of thought which they evince. Modern travellers have amply confirmed
the opinions of the father of history, and we now know that but for its
central river, Egypt would be a vast dreary expanse of arid sand like
the neighbouring deserts of Lybia. The Nile, by annually inundating
the country, deposits over it a stratum of rich loam, and thus not
only waters the land, but continually renews the soil. The sediment
in this way brought down has gradually encroached upon the waters of
the Mediterranean, being heaped up at the river mouth into shifting
sand-banks, islets, and great tracts of low, swampy ground, slightly
raised above the sea-level. Through this series of silting deposits,
the river sends a number of branches, often winding in labyrinthine
convolutions, and ever changing their course, by wearing away the silt
at one place, and throwing it down at another. The area traversed by
the mouths of the Nile was called by the Greeks the Delta, from its
similarity in form to the Greek letter, and the name has since been
given to all such fluviatile deposits, whether they have this general
form or not.
[Footnote 67: _Euterpe_, 5.--His words are very emphatic. "To one of
ordinary intelligence, who has not heard of it before, but sees it,
Egypt is manifestly land acquired by the inhabitants, and a gift from
the river--δωρον τον ποταμον." The 10th and 12th chapters of the same
book deserve especial study for the admirable inductive style in which
the historian compares the phenomena observable in Egypt with what were
well known as the results of river action in other lands. The passages
might be quoted word for word in the most rigid scientific argument of
any modern geologist.]
The sediment annually deposited by the Nile varies in thickness in
different years. The mean thickness of the annual layers at Cairo has
been calculated not to exceed that of a sheet of thin pasteboard, so
that "a stratum of two or three feet must represent the accumulation
of a thousand years."[68] Such thin laminæ must resemble greatly some
of the more fissile shales in the Carboniferous system, which were,
perhaps, formed by as slow a process, and in their aggregate depth
probably took many thousand years to accumulate. But those fluviatile
depositions of the Nile vary little in kind, for when cut through they
are found regularly stratified down to their base, which rests upon the
great underlying sand. They show us how the argillaceous seams of the
coal-measures may have originated; but the diversity of character in
these Carboniferous rocks indicates a more varied kind of sediment, and
probably more rapid and active transporting currents. A closer analogy
to such a condition of things meets us on the shores of the New World.
[Footnote 68: Lyell's _Principles_, p. 262.]
The Mississippi, so magnificent in all its proportions, has raised
a delta which covers a tract of about 14,000 square miles, equal to
almost half the area of Ireland. The lower parts of this delta are
formed of low, shifting banks, traversed by innumerable streams that
diverge from the main river, and alternately throw down and remove vast
quantities of earthy sediment, intermingled with rafts of drift-wood.
These swamps are covered with a rank growth of long grass and reeds,
and for about six months of the year are more or less submerged below
the waters of the river, while liable at the same time to continual
inundation and encroachment from the sea. The higher parts of the
delta, though also subject to a similar periodical submergence,
nourish a more luxuriant vegetation. Vast tracts of level sandy soil
are densely overgrown with pine, which is used extensively for making
pitch. Large districts of the swampy ground are covered with willows,
poplars, and thickets of the deciduous cypress, an elegant tree that
rises more than 100 feet above the soil. When in hot seasons these
swamps get dried up, "pits are burnt into the ground many feet deep,
or as far down as the fire can descend without meeting with water,
and it is then found that scarcely any residuum or earthy matter is
left. At the bottom of all these 'cypress swamps' a bed of clay is
found, with roots of the tall cypress, just as the underclays of the
coal are filled with stigmaria."[69] In this way a thick accumulation
of vegetable matter goes on forming for years, until either the river
changes its course, and inundating the swamp gradually covers it over
with sand and mud, or until, owing to oscillations of the earth's
crust, the district is either permanently submerged, so as to be silted
over, or elevated to nourish a new and different kind of vegetation.
[Footnote 69: Lyell's _Elements_, p. 386.]
That such changes have taken place in the past history of the river we
have several interesting proofs. Thus, owing to the great earthquakes
of 1811, 1812, an area of more than 2000 square miles was permanently
submerged.[70] Since then it has gone under the name of the "Sunk
Country;" and Sir Charles Lyell, who visited the locality in 1846, that
is, thirty-four years afterwards, tells us that he saw innumerable
submerged trees, some erect, others prostrate. Now, it is easy to see
how such an area may, when the climate suits, become the receptacle of
vast accumulations of peat, which, by pressure and chemical action,
will ultimately pass into coal. If we suppose the submergence carried
on more rapidly at some periods, the plants might have been unable to
keep pace with the ever-increasing inroads of sand and mud. In such
cases the layer of vegetation would become eventually entombed beneath
succeeding deposits of earthy matter. Were the amount of sediment thus
thrown down sufficient in the end to counteract the downward motion of
the earth's crust, and so raise the bottom of the river or lake to the
level of the water, vegetation would spring up afresh and clothe the
new raised surface as densely as in former years. This alternation,
according as the amount of sinking or the amount of sediment
predominated, might go on for thousands of years, until a series of
strata many thousand feet thick were accumulated, and tranquilly
carried down bed after bed below the level of the waters.
[Footnote 70: See Sir Charles Lyell's _Second Visit to United Stales_,
chap, xxxiii.]
It is interesting to know that the case supposed here has actually
been realized in the delta of the Ganges. Some years ago an Artesian
well was attempted to be made near Calcutta, and the auger was sunk
to a depth of 481 feet.[71] The material passed through consisted of
sand, clay, and nodules of argillaceous limestone, and at various
depths, from 50 to 380 feet, several seams of decaying wood and peat
were found, along with bones of various animals, such as deer and
fresh-water tortoises, and fragments of lacustrine shells. Each of
these vegetable layers evidently formed at one time a forest-covered
swamp like those of the surrounding delta at the present day; and
hence it follows, that during the accumulation of the Gangetic delta,
the ground in that locality must have undergone a depression of more
than 300 feet, and that this sinking has been interrupted by slight
elevations, or by periods when the ground remained stationary, so as
to admit of a dense and prolonged growth of vegetation, at successive
intervals, upon the swampy flats and shifting islands. The general
appearance of these old forests is pretty well shown by the mangrove
swamps along the mouths of the river. These trees flourish in dense
jungles on the banks, and extend even below high water mark, being
covered in places by shell fish. So that were these maritime parts
of the delta inundated by the ocean, and buried beneath a mass of mud
and silt, the peaty layer that would be formed would display trunks of
trees still occupying their original erect position, and spreading out
their roots in the clay below, exactly as the sigillaria is found to do
in the coal-seams of the carboniferous rocks, while clustered round the
carbonized stems, or scattered among the decayed leaves and branches,
there might be detected limpets and barnacles (as lingulæ and pectens
occur in the coal-seams), showing, by their mode of occurrence, that
they lived and died upon the spot.
[Footnote 71: See Lyell's _Principles_, p. 280.]
If my reader will now suppose this sand of the Indian river to be
hardened into sandstone, the mud in like manner compressed into
shale, and the peat beds chemically altered into coal, can he fail to
perceive the striking analogy between the section thus displayed and
those already given from the Mid-Lothian and Cape Breton coal-fields?
The differences between the ancient and modern strata are not in kind
but in degree. The Scottish series reaches to more than six times the
thickness of the Indian one, and the coal-seams in the one exceed in
individual thickness the peat-beds in the other. We must remember,
however, that the climate of Hindustan is not remarkably favourable to
the accumulation of vegetable matter, the heat being so great that the
plants decay almost as rapidly as they grow. And it should likewise
be borne in mind, that were the conditions of subsidence and of the
gradual accumulation of sedimentary matter to continue even in the same
ratio as heretofore, the Ganges might, in the course of ages, heap up
a series of stratified sands, clays, and peat-beds, many thousand feet
in thickness, and many thousand square miles in extent, rivalling,
or perhaps surpassing in depth, the largest coal-field in the world.
The parallelism between this delta and an ordinary coal-field holds
singularly close, not merely as regards the nature of the stratified
deposits. The alluvial plain of Bengal has undergone a process of
subsidence to an unknown depth, whereby successive areas of terrestrial
vegetation have been carried down to be entombed beneath fluviatile
sand and mud. It is likewise subject to the more sudden operation
of earthquakes, whereby large tracts of country become permanently
altered, and changes are effected on the direction, rapidity, and
detritus of the streams. It is, moreover, liable to wide-spread
inroads of the sea, which sometimes covers cultivated districts to
a depth of several feet, laying waste the fields and destroying the
inhabitants. These and other features help us to understand the origin
of such vast masses of sedimentary strata as those of our coal-fields,
where terrestrial, fluviatile, and marine remains alternate in rapid
sequence, or sometimes occur together.
The origin of the constant succession of coal seams, sandstones, and
shales, of the Edge series may be thus accounted for. The area of
Mid-Lothian formed part of a great delta, which, like that of the
Ganges, was undergoing a gradual subsidence during the Carboniferous
era. The rate of this movement probably varied at different times, and
might even be occasionally interrupted by short periods of elevation.
When the ever-increasing accumulations of silt brought down by the
river reached or nearly reached the surface of the water, they would
become the site of wide tracts of swampy vegetation that flourished
for hundreds or thousands of years. Eventually, however, these
jungles, invaded by the changing currents of the river, were buried
beneath a thick deposit of fluviatile sediment, or more probably the
vegetation might become unable to keep pace with an accelerated rate
of submergence, and the forests would then be tranquilly carried
down beneath the water, and soon covered over with sand and mud. The
detrital matter might in like manner continue to be deposited over the
sunk forest for many years, perhaps centuries, until the muddy bottom
again reached the surface, and once more waved green with sigillariæ,
calamites, and lepidodendra. Another long interval might here elapse,
in which a thick bed of vegetable matter might accumulate, much after
the manner of the formation of peat among the bogs and mosses of our
own country. The periodical inundations of the river probably gave
rise to wide marshes and lagoons, often tenanted by lacustrine shells,
and thickly overgrown with aquatic vegetation. The decaying plants
decomposed the red ochreous matter with which the water was charged,
and re-deposited it among the mud and rotting leaves at the bottom as
a carbonate of iron. Such ferruginous accumulations, often entombing
fern-stems and other plants, with scales and teeth of ganoidal fishes,
sometimes _conulariæ_ and _lingulæ_, and, in certain localities, whole
acres and miles of fresh-water shells, are known now as our _clay-band_
and _black-band ironstones_. We can easily conceive that, in shallower
parts of the lagoons, a dense growth of marshy plants might spring
up, preventing any deposition of iron, and when the whole came to be
covered over with later accumulations of sand or mud, the deeper parts
of the old lake would be covered with a seam of ironstone, and the
shallower portions would display a bed of coal. In some such way we
may account for the frequent passage of ironstone into coal, and coal
into ironstone in many of our coal-fields. If undisturbed by the ever
changing currents of the river, these wide expanses of marsh and lake
might continue for many long years, the constant evaporation being
counterbalanced by continual supplies of water from the main stream.
Eventually, however, owing perhaps to another period of more rapid
submergence, the water gained the ascendency, and once more rolled over
prostrate stems and matted thickets of ferns, that sank slowly down
beneath a deepening sheet of sand and mud. Often, too, the sea must
have flooded, perhaps for years, the flat delta-lands, carrying with it
its own productions, such as the lingulæ and cardiniæ, which we find
among the coal seams. And thus the process went on during the long ages
of the Carboniferous system. Forest after forest spread its continuous
mantle of green athwart the low swampy lands of that old delta, and
each in succession foundered amid the muddy waters, now of the ocean
and now of the river, that strewed over its site a rich detritus which
went to form the soil of new jungles and forests.
The Edge series measures from 800 to 900 feet in depth, so that the
depression must have been carried on till the forest that once grew
nearly on the sea-level had sunk 800 feet below it This process was
undoubtedly a very slow and tranquil one. Yet geologists used to regard
these frequent changes of sedimentary matter as so many proofs of
repeated catastrophic submergences, when the ocean came rolling over
the land, prostrating forests, uprooting the hugest trees, and leaving
the scattered bones and scales of fishes amid vast accumulations of mud
and sand, where but lately there had bloomed a luxuriant vegetation.
But the sober and diligent student of geologic fact will read in these
rocks no such record of cataclysms. He will see in them evidences of
the same gradual and sure operation which marks the processes of Nature
at the present day. He will note how during a tranquil and probably
imperceptible submergence of the river-bottom, forest after forest
sprang up, flourished perhaps for ages, and eventually settled down
beneath the waters of the river and sometimes of the ocean, amid ever
increasing accumulations of mud and sand. Musing on these ancient
changes he will be lost in wonder at the immense duration of the period
during which they were in progress; and he will try in some measure
to realize the features of their scenery. He will picture the delta
with its ever-varying islets and sand-banks, its lakes and submerged
forests, its leafless trunks peering above the water and sticking along
the shoaling mud, and its crowded jungles that cover every drier spot.
He will cast his eyes to where the delta opens out into the ocean, and
mark how the waves encroach upon the mud-banks, cutting away what the
river has piled up, and washing the roots of gigantic trees that wave
their green coronal of fronds above, and overshadow the rippling of
the green sea below. He will try to thread the windings of the stately
river through brakes of ferns and calamites, and banks richly hung
with tree-ferns and sigillariæ, and then upward through dark shaggy
pine-woods, silent and gloomy, with the water creeping lazily through
the shade or dashing in white cascades over dripping rocks, and onward
still, far away among the distant hills till the fountainhead of the
great stream is reached, gushing from the splintered sides of some
lone rock, or pouring perchance out of the glimmering caverns of some
massive glacier high amid the regions of perpetual snow.
Many attempts have been made to estimate the amount of time which
some of our coal-fields may have required for their accumulation.
But so large a number of conjectural elements must necessarily enter
into such calculations, that the results come to be of very doubtful
value. By estimating the amount of sediment annually transported by
such rivers as the Ganges or Mississippi, we may ascertain how long a
mass of similar sedimentary strata would take to form under similar
conditions. And if our calculation had to do merely with such detrital
accumulations, we might hope to arrive at some approach to accuracy.
But besides these sedimentary strata, the formation of which must have
been wholly analogous to that of similar deposits at the present day,
we have to deal with the problems suggested by the coal-seams. We know
nothing of the climate of the Carboniferous period save what may be
conjectured from the analogy of existing climates; and in a question
regarding the accumulation of decaying vegetable matter climate is a
subject of the first importance. We are ignorant, too, of the rate of
growth peculiar to the carboniferous flora; and even if we hold that
it was probably rapid, the process of decay may have been equally
speedy, and so a forest might go on shooting up fresh trees as the old
ones rotted away, yet at the end of a thousand years there might be a
scarcely greater thickness of vegetable matter on the ground than at
the commencement. A seam of coal two feet thick might thus represent,
say the accumulation of a hundred years, and another of exactly the
same thickness might stand as the accumulation of a thousand years.
Until we know more of the vegetation and climate of the coal period,
the thickness of a coal-seam can hardly be held as a certain guide to
the lapse of time required for its formation.
For the sake of illustration, let me take the following fragment of a
coal-measure section:--
Shale, 20 feet.
Coal, 4 "
Fire-clay, 6 "
Sandstone, 40 "
Beginning at the bottom, we may compute the period of the forty feet
of sandstone variously, according to the river selected as the type
of a transporting agent. Tried by the standard of the Nile, all other
conditions being similar, such a deposit would require perhaps not
less than 14,000 years; by that of the Mississippi, 5000; and by that
of the Ganges, nearly 2000.[72] We come, then, to the superincumbent
fire-clay and coal, representing an ancient soil and the forest that
grew on it. The occurrence of these seams shows us that the river-bed
had become a swampy tract clothed with vegetation; but who shall
say how long it may have continued so? Like the sunk country of the
Mississippi, it may have been submerged, and to some extent cut off
from the sediment-transporting channels of the river, and thus, as
a vast lake, have nourished a prolific growth of marshy and aquatic
plants. If the temperature resembled that of our own country, the
growth of peaty matter, other circumstances being favourable, might
be comparatively rapid. If, however, as seems probable, the climate
were more warm and humid, giving rise to a more luxuriant vegetation,
and at the same time to a more rapid decay, a long interval might have
elapsed without adding materially to the thickness of the vegetable
accumulations, and the eventual entombment of peaty matter sufficient
to consolidate into four feet of coal, might be owing in some measure
to the submergence of the swamp beneath the waters of the river,
whereby a quantity of detrital matter was deposited that arrested the
process of putrefaction, and entombed the thickly matted plants which
were growing on the spot at the time. Hence, until we know more of
the conditions under which vegetation may accumulate at river-mouths
in such a climate as the coal plants are conjectured to have enjoyed,
calculations of the amount of time required for the formation of a
great series of coal-bearing strata must be regarded as premature.
In the present instance, we can but affirm that the growth of the
four-foot coal-seam probably occupied many long years, even at the most
rapid rate of accumulation known to us. The forest-covered swamp on
which the plants grew was eventually invaded by muddy detritus brought
down by the river; and during another period of indefinite extent--five
hundred years or five thousand years--fine mud continued to settle down
over the foundered forest, hardening eventually into twenty feet of
shale.
[Footnote 72: Some observers have pointed to the occurrence of vertical
and inclined trunks of trees in the Carboniferous sandstones, and
deduced therefrom what has seemed to them a triumphant argument in
favour of the rapidity wherewith our coal-fields must have formed. A
foundered tree, they say, sank with its heavy-laden roots among the
sand at the bottom, its stem pointing up into the water like the snags
of the Mississippi, so that the sand must have come rapidly down to
entomb the whole before it had time to decay, and thus thirty or forty
feet of sediment must have been deposited in a few years, perhaps even
months. But this is somewhat like a begging of the question. We have
yet to learn how long a water-logged trunk will resist decomposition.]
The Edge coals of the Mid-Lothian coal-field are succeeded by a group
of sandstones and thin shales, with three or more seams of limestone.
This group of strata, which we may call the Roslyn Sandstone Series,
reaches a thickness of from 1200 to 1500 feet, and serves as a middle
zone to divide the Edge coals below from the Flat coals above. It
contains only a few thin laminations of coal, and these chiefly at its
upper and under portions. Such a great intercalation of beds, without
coal-seams, points, we might readily conjecture, to some change in the
physical conditions of the ancient delta. The nature of this change can
be easily made out from an examination of the rocks, and the reader
will see that here again we are indebted to fossil remains for the most
conclusive and satisfactory evidence of these old physical revolutions.
The absence of coal-seams suffices to indicate that during the
formation of the middle group that part of the delta occupying the
site of Mid-Lothian was continually submerged, and never rose to the
surface so as to allow a covering of vegetation to form upon it.[73]
The large beds of sandstone prove a continued transport and deposition
of detritus during undisturbed periods of considerable length. The
intercalations of shale, pointing to local changes in the currents or
other modifying causes, are usually of small thickness and extent,
while the sandstone beds sometimes attain a depth of 150 or 200 feet,
and extend over wide areas of country. So far these mechanical rocks
indicate the deposition of sand and mud under water, but whether at
river-mouth or sea-bottom is left uncertain. From the fossil remains,
however, we learn that the deposition took place in the sea, but at
no great distance from land; in other words, the area of Mid-Lothian,
which, during the accumulation of the edge coals, had been alternately
clothed with vegetation and inundated by the river, sank down many
fathoms, so that the sea rolled over it and all its submerged forests.
The proof is two-fold, first, from the character of the organic remains
in the limestones; and second, from that of those in the sandstones and
shales.
[Footnote 73: Of course, this deduction is founded, as the reader
will notice, on the assumption that we have now the series, as it was
deposited, and that no peaty swamp or forest was denuded away, and its
site occupied by sand and silt. But the assumption is rendered probable
from the conditions of formation indicated by the Roslyn group.]
In some of the streamlets that flow into the beautifully wooded vale
of the Esk, south of Penicuik, these limestones can be well seen,
worn in the water-channel, or crusted over with moss along the banks.
Their organisms are singularly abundant, and consist of cyathophylla,
encrinites, spirifers, producti, &c., all exclusively marine. In a
picturesque brook that falls into the Esk near a saw-mill in the
grounds of Penicuik House, I have seen the little cup-corals clustered
by dozens on the weathered rock, showing their delicate striated
wrinkles in high relief among the scattered valves of productus and
innumerable joints of the stone-lily. They were all well preserved, and
in their grouping and general appearance differed in no respect from
similar organisms in the mountain limestone of Roman Camp Hill. The
inference to be drawn from them must accordingly correspond with what
has been deduced from the mountain limestone fossils, viz., that they
mark the site of a sea-bottom which remained free from mud and sand for
considerable periods, during each of which there abounded corals and
shells, whose exuviæ went to form several seams of limestone. But that
this sea-bottom was at no period very far distant from land, is proved
by the drifted plants that occur in the sandstones and shales both
below and above, and which often show so little trace of maceration,
that we can hardly believe they were carried far, or floated for a long
while previous to being enveloped in the sand or mud at the bottom. I
have never detected vegetable remains in the limestones themselves, but
there seems no reason why they should not be found there.
One of the most remarkable and difficult phenomena presented by these
limestones is their great persistency. I have traced them over a
large part of Mid-Lothian, from the highly inclined beds at Joppa to
the contorted and faulted strata near Carlops. I have found them,
too, in many parts of West-Lothian and Stirlingshire, from the sea at
Borrowstounness southwards into Lanarkshire. They likewise occur in
Fife, and seem to sweep away through Lanark and Ayrshire. The area
in which I have found them cannot be much under 700 square miles,
yet they are probably spread over a much greater extent of country.
Throughout this region they appear to continue on the whole at pretty
much the same vertical distance from each other, and average three or
four feet thick each. They vary in number, three being found in parts
of Mid-Lothian, in other parts only two. Throughout West-Lothian there
seem to be but two seams in the middle or moor-rock series, and the
same two seams are found passing over into Perth near Culross. There
are differences, too, in the structure and composition of the seams,
one running sometimes as a single bed of dull blue limestone, and
then gradually splitting up into three layers of a greyer and more
earthy texture, with soft shale between them. But making all these
abatements, the observer cannot fail to be struck with the general
regularity and continuity of these limestones. And the fact becomes
all the more remarkable when we consider the great irregularity, and
continual intercalations, and repetitions of the strata, both above and
below. Marine beds are usually persistent over large areas, especially
where extensively developed. As they decrease in thickness, their
continuity for the most part lessens, so that the rule is, on the
whole, a safe one, the thinner any particular stratum, the less likely
are we to trace it to a considerable distance. Yet, not only are these
Mid-Lothian limestones thin, but they occur in regular sequence among
a set of continually alternating and very irregular beds, and extend
over several hundred square miles of country. And this, too, not in a
single seam, but in two, three, or even more, so that the difficulty of
accounting for such intercalations is proportionately increased.
We have seen above that the area of a delta is often partially
submerged below the sea, and that such changes may become of the most
marked kind where the country is liable to be depressed by earthquakes.
There can accordingly be no difficulty in understanding how the ancient
carboniferous delta of Mid-Lothian may have likewise subsided. But the
limestones are unmistakable evidence that not only was the area of
the delta submerged, but that for a while no sediment was deposited
over it, and hence marine animals peculiar to clear water flourished
so long and so abundantly as to form by their remains several beds of
limestone. Had these beds been merely local we might have regarded them
as having been deposited in lagoon-like portions of the delta, shut out
from the detrital matter of the river on the one side and open to the
sea on the other. But their wide extent and nearly uniform thickness
preclude such a supposition. The following explanation appears to me
the most probable:--
After the series of the Edge coals had been brought to a close, the
coal-fields of Scotland underwent a complete submergence below the sea.
This depression was probably very gradual, yet more rapid than that
long-continued one which had been going on during the earlier part of
the Carboniferous series, and the consequence of this greater rapidity
was to prevent the growth of stigmaria swamps or reedy jungles, by
keeping the alluvial surface continually sunk to some depth below the
water. The amount of subsidence until the deposition of the lowest
limestone may not have been great, but even a slight depression would
tell vastly on an area of flat delta land. Mud banks would be brought
down into the region of waves and surface-currents, and speedily be
spread out over the floor of the sea. Forest-covered islands would in
like manner be levelled down, and their trees sent drifting seaward
or submerged amid the re-formed silt. Thus altered, the delta would
sink below the sea, and the sediment borne down by the river would
be scattered out over the older deposits as a slowly-forming sheet.
By degrees this detrital matter must have been carried less and less
farther out to sea; in other words, the area of deposit or delta must
have crept gradually nearer to the land--a result owing partly to the
recession of the ancient coast-line, and partly perhaps to a greater
amount of depression inland than at the coast, which would of course
lessen the velocity of the streams and cause them to deposit their
burden of sediment at higher levels than before. The consequence
of this retreat of the delta from the sea would be to purify the
water over the site of the old swamps, and render it fitted for the
habitation of corals, molluscs, and other marine animals. A medium thus
prepared would not be allowed to remain long untenanted, and so we find
that it came to be densely peopled with the organisms peculiar to such
a station. Stone-lilies, cup-corals, net-like bryozoa, molluscs of many
kinds, and large predatory fish, swarmed in these old waters, and their
calcareous shells and skeletons are now broken up by the quarryman and
the collier as hard compact limestone.
After these animals had lived and died in successive generations,
perhaps for thousands of years, the downward movement of the earth's
crust seems to have ceased for a while or to have become greatly less.
The effect of this would be just to reverse what had been previously
done, especially if a slight elevatory movement took place. The streams
would in such circumstances descend from the uplifted ground with
renewed velocity and transport their detritus to gradually increasing
distances. The muddy and sandy sediment thus borne seawards would
slowly silt over the coral-banks at the bottom, and in conditions so
ungenial the organisms would dwindle down and finally die out. A great
thickness of sand and mud would be spread out over their remains so
long as the currents from the land continued to carry sediment out to
sea, and thus probably originated the sandstones and shales superposed
above the lowest limestone.
Eventually the old steady downward movement returned, and with it the
corals and stone-lilies. The detritus again sank to the bottom much
nearer the land, forming great banks and shoals that choked up the
river-mouth. Seaward the water regained its purity, and the bottom
once more swarmed with living things. Another lapse of many thousand
years may have here intervened during which the marine exuviæ gathered
into another seam of limestone, until again the process of subsidence
either ceased for a time, or what is perhaps more probable, became
considerably feebler. Detrital matter began to creep seaward as
before, and eventually entombed the corallines and crinoids to a great
depth. The calcareous bed thus formed is the second limestone, and the
superincumbent silt-beds represent the sandstones and shales that rest
above it.
In some such way as this does the Roslyn sandstone series appear to
have originated. I have indicated what seems to have been the main
features in the process, but it was probably a very complex one. There
may have been a great many oscillations of level of variable effects,
some of them raising the disturbed area to a much greater height at
one point than at another. This inequality would of course produce
marked effects along a low flat country such as that at the mouth of a
great river. New currents would be produced and the direction of old
ones changed; great shoals and banks of silt would be worn down only
to be thrown up again at some new point, where another oscillatory
movement would expose them afresh to destructive denudation. The
variations in the amount of elevation and depression would likewise
modify the transport of detritus to the sea, and give rise to a varied
and ever-changing sea-bottom. In short, the alternations and variations
must have been endless, for to the ordinary multiplied interchanges of
a delta we must add those induced by a constant and unequal oscillation
of the earth's crust.
The Roslyn sandstone series comes to a close, and passing onward in
ascending scale we meet with another great group of coal-bearing
strata. They occupy the central area of the Mid-Lothian coal-field,
and from their gentle inclination as compared with the lower strata
that rise up from under them on either side of the basin, are known as
the _Flat Coals_. Their total thickness--that is to say, all that has
escaped denudation--amounts to a thousand feet or more. They consist
chiefly of sandstones, shales, ironstones, and fire-clays, with from
twenty to twenty-five seams of coal, of which sixteen are thick enough
to be worked. Their similarity to the Edge coals below points to a
similarity in the conditions of formation. The frequent alternations
of sandstone and shale show how the delta gradually pushed outwards
again and re-occupied its ancient site above the successive forests of
the Edge series and the successive coral-beds of the Roslyn group. The
coal-seams indicate the further progress of the detrital accumulations,
and the eventual formation of vast swampy flats that nourished a rank
growth of stigmariæ, and tracts of drier ground waving with ferns, and
shadowed by the spiky foliage of the club-moss and the broader fronds
of the tree-fern.
The Flat coals are not succeeded by any other palæozoic strata. Above
them stretches the drift already described: sometimes in the form
of a stiff blue clay resting on a striated rock-surface; sometimes
as a coarse gravel containing fragments of all the rocks in the
neighbourhood; and sometimes as a fine white sand diagonally laminated,
and often showing dark partings of coal-fragments. From the section
given above (Fig. 32) at p. 196, the reader, will see that as the
upper limit of the Flat coals is formed by the drift, a large part of
that series may have been borne away by denuding agencies. Had there
been even a seam of limestone above them, it would have sufficed to
show their true thickness, for we should then have seen, that how
much soever had been removed in later times from above the limestone,
nothing had been removed from below it; and so it would mark the true
original limit of the series. We cannot now tell how much thicker the
upper part of the Mid-Lothian carboniferous system may have been.
Probably, during the long ages that intervened between palæozoic and
post-tertiary times, many hundred feet were borne away and carried to
other sites, there to grow up into new islands and continents, clothed
with other types of verdure, and peopled by other races of animals, and
fitted to become, in a long subsequent period, the dwelling-place of
man.
In fine, the evidence of these ancient changes in the history of the
Mid-Lothian coal-field is derived, as we have seen, from two sets of
facts; first, those of a mechanical, and, second, those of an organic
kind--the one class explaining and confirming the other. Beginning our
investigation at the horizon of the Burdiehouse limestone, we saw the
curtain rise slowly from off a wide estuary, in which there gambolled
large bone-covered fishes, while huge pine-trees--branchless and bare,
seed-cones, fern-fronds, and twigs of club-moss, floated slowly away
out to sea. The panorama moved on, and brought before us the ocean-bed
of the Roman Camp limestone, with its groves of stone-lilies and
bunches of coral; its tiny shells moored to the bottom, or creeping
slowly athwart the limy floor, or spreading out their many arms, and
rising or sinking at will. This picture passed slowly away, and then
came the delta of the Edge coals, with its sand-banks and ever-shifting
currents, its stigmaria swamps, and its forest-covered islets. We saw
the delta gradually sink beneath the sea, and corals and stone-lilies
cluster thick over its submerged area, to form the limestones of the
Roslyn group. Again, the mud-bars of the river crept out to sea, and
tangled forests waved green as of old, washed by the sea or inundated
by the river. How this last period came to a close, we shall probably
never know, and have no possible means of conjecturing. We pass at one
step from the ancient era of the coal to the comparatively modern one
of the drift--from a verdant palæozoic land, to an icy post-tertiary
sea. It is like a leap in history from the days of Pericles and Aspasia
to those of King Otho, or from the tents of Runnymede to the Crystal
Palace of Sydenham.
CHAPTER XII.
Trap-pebbles of the boulder--Thickness of the earth's crust
unknown--Not of much consequence to the practical
geologist--Interior of the earth in a highly heated
condition--Proofs of this--Granite and hypogene
rocks--Trap-rocks; their identity with lavas and
ashes--Scenery of a trappean country--Subdivisions of
the trap-rocks--Intrusive traps--Trap-dykes-intrusive
sheets--Salisbury Crags--Traps of the neighbourhood of
Edinburgh--Amorphous masses--Contemporaneous trap-rocks of
two kinds--Contemporaneous melted rocks--Tests for their age
and origin--Examples from neighbourhood of Edinburgh--Tufas
or volcanic ashes--Their structure and origin--Example
of contemporaneous trap-rocks--Mode of interpreting
them--Volcanoes of Carboniferous times--Conclusion.
In the previous pages, allusion has been made to the trap-pebbles
imbedded in the boulder, to the various forms of decay exhibited by
granitic and trappean rocks, and to the elevation and depression of the
solid crust of the earth. Will the reader bear with me for but a few
pages more, while I seek to indicate one or two points of interest in
a branch of geology that would abundantly reward a diligent observer?
Since the days of Hutton, the investigation of what are called
_igneous_ rocks has fallen somewhat into the background, and geologists
have given themselves, perhaps too exclusively, to the study of organic
remains, so that while the palæontology of the British islands has
enjoyed an extensive exploration, but little has been done towards the
elucidation of our igneous formations and their accompanying phenomena.
Much remains to be accomplished, even in those districts usually
regarded as in a manner thread-bare, and he must be but an indifferent
observer who cannot add a few gleanings to the general stock of
information upon this branch of British geology.
Many conjectures have been formed, and many theories propounded, as
to the nature of the internal parts of our globe. Some have supposed
that there is an outer solid film or crust, some ten or twenty miles
thick, enveloping a vast ball of intensely heated matter; others have
attempted to show that the interior must be nearly solid throughout,
with, however, great lakes, or vesicles of gas and melted rock,
somewhat after the fashion, we may suppose, of the oil-holes in a
Gruyère cheese. But whether the heated material occupy the whole or
only parts of the internal area, is not of much consequence to the
practical geologist; he is content to believe that it exists, and in
sufficient quantity, too, to produce the most momentous changes on
the surface of the earth. We see the effects of this subterraneous
agent in earthquakes and volcanoes, and the geologist can tell us
of similar, as well as of other changes, effected by it during past
ages. Granite hills, and mountainous districts of mica-slate and
gneiss, bear evidence of what is termed _metamorphism_--a change in
the mineral structure of rocks, believed to have taken place through
the agency of heat deep in the interior of the earth; for no analogous
appearances have been detected in progress at the surface. Such rocks,
known as _metamorphic_, or _hypogene_, still form a difficult problem,
not likely to be satisfactorily solved until the chemist shall have
thoroughly investigated the subject; for it seems likely to be found,
after all, that long-continued chemical action, without a very alarming
degree of heat, may have produced even the most intense metamorphism.
But dropping this part of the subject, in which so much yet remains
to be discovered, let us look for a little at another branch of the
geologist's evidence, where we meet with no such hampering hypotheses
and doubtful conjectures, namely, the _trap_-rocks.
Every one knows that basalt, lava, pumice, scoriæ, and ashes, are
the various matters ejected from volcanoes. When these materials are
found interstratified among the various geological formations, they
are termed _trap_-rocks,--a name derived from the Swedish _trappa_, a
stair, in allusion to the step-like or terraced appearance which they
often present. They are of all ages, having been detected in the lower
Silurians of Wales, and in the deposits of all subsequent periods up
to the volcanic eruptions of the present day; thus evidencing, that
from the remotest times there have been Ætnas and Vesuvii slumbering
perhaps for ages, and then awakening to lay the surrounding districts
in ruins. I have already said that the rocks from which the geologist
has to compile his history, are mostly relics of the sea; hence most of
the trap-rocks which he meets with in his explorations are the products
of submarine eruptions. Far away down among the Silurian rocks, he can
trace the floor of a primeval ocean thickly covered with stone-lilies,
trilobites, and molluscs, and in following it out he marks how ashes
and lapilli, ejected from some submarine orifice, settled down amid the
organisms and well-nigh destroyed them, while at other times streams of
molten matter were poured out along the sea-bottom, and hardened into
masses of solid rock. He sometimes even encounters what seems the vent
whence these eruptions proceeded, filled up now by a boss or plug of
hardened trap, but he never can detect any trace of land. Some of these
oceanic volcanoes may, like Graham's Island in the Mediterranean, have
raised their tops above water, sending clouds of steam and cinders far
and wide through the air, but the waves would eventually wear down the
new-born land, and scatter its broken fragments along the floor of the
sea. Among the carboniferous rocks of Scotland, however, we meet with a
different condition of things. There, too, we can trace out submarine
lava-streams, and mark how showers of ashes destroyed the delicate
organisms of the deep; but we encounter, besides, undoubted traces
of a land, not parched and ruinous as though the igneous forces had
laid it waste for ever, but thickly clothed with vegetation of a more
luxuriant type than that which clusters over Vesuvius and Calabria, or
lies spread out across the "level plains of fruit-teeming Sicily."[74]
We have looked at the plants and animals of the Carboniferous era; its
rivers and deltas; its slow elevations and depressions of the ground.
It may, perhaps, complete the picture of that ancient period, if we
examine, though but briefly, its igneous eruptions, the more especially
since these may be regarded as, to a considerable extent, typical of
trap-rocks belonging to every age and every country.
[Footnote 74: Της καλλικἁρπον Σικελἱας λενροὑς γὑας. Æsch. _Prom.
Vinct._ 369--a passage graphically descriptive of an ancient eruption
of Ætna.]
Unless when deeply buried beneath drift-sand and clay, trappean regions
usually possess scenery of a marked kind. A green undulating country
stretches out as far as the eye can reach, diversified here and there
with bold abrupt crags and conical hills. The lower grounds show in the
winter season their rich brown loam, that waxes green as the spring
comes on, and ere summer's close spreads out its heavy crops of golden
grain. The higher ridges are for the most part thickly wooded, yet
the soil is often scanty, and, among the white stems of the beech, or
the matted roots of the fir and the elm, we may not unfrequently see
the rock protruding its lichen-crusted face, mottled with mosses and
liverworts, while some sluggish runnel collects in stagnant pools,
or trickles over the blocks with a thick green scum. Sometimes the
hill has never been planted, but stands up now, as it has done for
centuries; its western face craggy and precipitous, with bushes of
sloe-thorn and furze, and stray saplings of mountain-ash clinging to
the crevices, while its eastern slope sinks down into the rolling
country around with a green lumpy surface, through which, at many a
point, the grey time-stained rock may be seen. The whole district
suggests to the fancy a billowy sea, and, as one casts his eye from
some commanding hill-top athwart the wide expanse of hill and valley,
sweeping away in endless undulations, he is apt to bethink him of
some day far back in the past, when the verdant landscape around lay
barren and desolate, while the solid earth rocked and heaved in vast
ground-swells like a wide tempested ocean. Such is the aspect presented
by some of the more trappean regions of Scotland. But the origin of
this kind of scenery must be ascribed to the effects of denuding
currents in scooping out the softer strata into clefts and valleys, and
leaving the harder trap-rocks in prominent relief, rather than to any
great inequality of surface produced by the eruption of igneous matter;
for we shall find that the throwing out of sheets of lava and showers
of volcanic ashes was often a very quiet process after all.
Trap-rocks generally may be variously classified according to the
aspect under which we view them. Mineralogically they are _augitic_,
when the mineral _augite_ enters largely into their composition;
_hornblendic_, when the _augite_ is replaced by _hornblende_; and
_felspathic_, where _felspar_ forms the most marked constituent.
The first class includes all the dark homogeneous compounds called
_basalts_; the second, the hornblendic _greenstones_, or _diorites_;
and the third, the _felstones_, _porphyries_, and _tufas_.
Geologically, they are _beds_ when they are interstratified with the
contiguous rocks; and _dykes_ or _veins_ when they penetrate them like
walls, or in an irregular manner. The former class may be either of
the same age with the rocks among which they lie, or of a later date,
just as in a pile of books the centre one may either have been placed
there originally with the rest, or thrust in long afterwards. The
latter class must always be later than the rocks which they traverse,
for it is plain the rocks must have been in existence before trap-dykes
and veins could be shot through them. Hence geologists are accustomed
to speak of contemporaneous and subsequent trap-rocks: the one list
including all the tufas, and those melted rocks which can be shown to
have been erupted during the time when the limestones, sandstones, or
shales around them were forming; the other embracing all the dykes and
veins along with those beds of melted rock which have been intruded
between the strata. These and other distinctions will be better
understood from a few examples collected chiefly from the carboniferous
district of central Scotland.
The trap-rocks seen there exhibit a wide range of structure, texture,
colour, and general aspect. There are two pretty marked kinds--the
augitic or hornblendic, and the felspathic; the former being usually
of a more or less crystalline aspect; the latter, commonly dull, and
often without any crystals.[75] In the augitic traps, the crystals
are sometimes of large size and well-defined, so that the rock could
hardly be distinguished at first sight from an ordinary grey granite,
while at other times, and not unfrequently even in other portions of
the same mass, the stone assumes a black appearance without distinct
crystals. The former variety would be called a _greenstone_, the
latter a _basalt_; the chief components in either case being felspar
and hornblende, or felspar and augite, with a variable admixture
of other minerals, the shade of colour varying from a pale blue or
leek-green, through the different hues of grey, to a deep velvet black.
There are other traps, however, consisting entirely, or nearly so,
of felspar, whence they are known as _felstones_. Such rocks enjoy
a wide range of colour, some of them being pure white, others of a
bluish grey or dingy brown; and they may be seen graduating from a
pale yellow, or flesh-colour, to a brick-red or deep purple. When a
trap displays distinct disseminated crystals, usually of felspar, it
becomes a _porphyry_; when it shows rounded cavities, like those of
furnace-slag, it is said to be _vesicular_; and when these globular or
almond-shaped cavities are filled with carbonate of lime, chalcedony,
or other minerals, the rock forms an _amygdaloid_. Such peculiarities
of structure indicate to some extent the origin of the mass, and may
be found in any kind of trap. Thus we have porphyritic greenstones,
basalts, or felstones, and the same rocks may be likewise vesicular
or amygdaloidal. Some of them, such as many greenstones, display on
weathered surfaces that curious spheroidal structure already alluded
to; others are built up into geometric columns.
[Footnote 75: This distinction, though a sufficiently safe one in
some localities, must not be held as by any means universal in its
application, the felspathic traps being often as crystalline in aspect
as the augitic, and the augitic, on the other hand, as dull as the
felspathic.]
Such peculiarities of composition and structure form the basis of
a mineralogical classification of the igneous rocks, which is of
use in working out the geology of a district. The most convenient
subdivision for our present purpose, however, is that which proceeds
upon the origin and mode of occurrence of the trap-rocks. Viewed
thus, they resolve themselves into two great groups, the _intrusive_
and _contemporaneous_, both of which contain greenstones, basalts,
&c.,--the sole distinction between those of the one class and those of
the other, being the relation of age and mode of occurrence which they
bear to the surrounding rocks.
I. The _intrusive_ traps occur in the form of walls and veins,
sometimes in that of flat parallel beds, and often as huge amorphous
masses, to which no definite name can be given. But whatever shape they
may assume, they generally agree in presenting well-marked features,
whereby their origin can be readily ascertained. The rocks through
which they pass are more or less hardened, often contorted, and
sometimes traversed by innumerable cracks and rents, into some of which
the trap has penetrated in the form of veins.
A trap-dyke is a long wall of igneous matter, cutting more or less
perpendicularly through the surrounding rocks. Sometimes these dykes
attain a breadth of many yards, and may not unfrequently be traced for
miles running in a nearly straight line over hill and valley, easily
recognisable by a long smooth ridge, with the rock protruding here and
there from below where the soil is thin. It is interesting to follow
out one of these long ramparts from its beginning to its close, and
mark how undeviatingly it cuts through the rocks. No matter what may
be the nature of the stone encountered, hard conglomerate, friable
shale, compact limestone, or jointed fissile sandstone, all are broken
across, and the right line preserved throughout. Nay, I have seen a
still more curious instance of this persistency, where the dyke ran for
four miles through a set of mountain limestone and lower coal-measure
strata, and several enormous sheets of greenstone and basalt. Even
when passing through these traps the dyke remained perfectly distinct,
its crystalline structure and external configuration presenting a
well-marked contrast with those of the surrounding eminences. Of
course it must have been formed after all the rocks through which
it passed. The sandstones and shales must have settled down long
previously on some estuary bed or sea-bottom; the corals and shells
of the limestones, and the matted plants of the successive coal-seams
must have lived and died, perhaps thousands or millions of years
before, and their remains have hardened into stone, ere the continuity
of the strata was broken across by the long deep wall of greenstone.
Trap-dykes are accordingly appropriately termed _intrusive_. They
have been intruded among and must always be later than the rocks in
which they occur. In tracing out their character, more especially in a
trappean district, such as that of Linlithgowshire, where they abound,
we soon find other evidence of their intrusive nature. Where they pass
through limestone, they sometimes convert it into a white saccharine
marble; shales they bake into a sort of porcelain or burnt pottery; and
sandstones become semi-fused into a hard homogeneous quartz-rock. Nor
are the changes confined to the rocks traversed; the dykes themselves,
along their sides, become fine grained and hardened; occasionally, too,
the colour alters from the usual bluish or greenish-grey to black, or
to a brick-red, or dull-brown, similar to that of the altered shale
and sandstone, of which detached portions may be found adhering to
the outer walls of the dyke, or even embedded in its substance.
The central portion of the dyke may thus be markedly crystalline,
forming what we should call a greenstone, while the outside parts,
where the trap comes in contact with the adjacent rocks, are fine
grained and homogeneous, so as to become a true basalt. Sometimes,
too, these exterior edges are highly vesicular and amygdaloidal,
detached fragments closely resembling the slag of an iron-furnace, and
occasionally the dyke presents a columnar arrangement, the ends of
the hexagonal or polygonal columns abutting against the sandstone or
other rock on either side, and losing themselves towards the centre in
the general mass of the trap. Where the strata traversed are broken
and jointed, the dykes which cut them through may be seen in some
places throwing out lateral veins that accommodate themselves to all
the irregularities of the fissures. These minor portions exhibit for
the most part the same leading features with the parent mass, and the
result of the whole is a general baking of the beds, with sometimes not
a little contortion, and an amount of irregularity and disturbance,
apparent at once to the most inexperienced observer. (See Fig. 34.)
If the reader will verify these statements by actual exploration in
the field, he will probably not be long in arriving at the following
conclusions: trap-dykes must once have been in a melted state, as is
shown by their vesicular cavities and divergent veins; this liquid
condition must have been attended with the most intense heat, as may be
gathered from the burnt and baked appearance of the contiguous rocks;
they have, for the most part, especially where of large size, risen
from below along previously-formed dislocations--a circumstance which
may be inferred from their persistency in a straight line through beds
of very different resisting power, for had the liquid matter forced
a way for itself, it would have squirted between the beds along the
lines of least resistance, and not directly and for miles across them;
and hence, trap-dykes must be regarded not as themselves the agents
in dislocating and contorting a district, but merely as signs of the
parent force at work below.
All the features of these trap-dykes here stated may be observed in the
central district of Scotland, among rocks of Carboniferous age. But he
who would study trap-dykes on the great scale without quitting Britain,
should visit some of the more trappean islands of the Hebrides. He
will there find them intersecting glen and hill-side, in an intricate
network, standing up through the heather like ruined walls, and running
often for considerable distances up bald cliff-line, and across
precipitous ravine. In some localities, among such limestone districts
as that of Strath, detached eminences may be seen with congregated
dykes coursing their sides and summits, while the heathy interspaces
are cumbered with grey and white protruding blocks of marble, that give
to these green knolls the aspect of old time-wasted abbeys with their
clustering tombstones. The magnificent sections laid open in these
localities by the action of mountain streams, and by the waves of the
Atlantic, leave the student of igneous rocks nothing to desire save a
long lease of leisure.
Another form frequently assumed by the intrusive traps, is that of
wide beds or sheets intercalated with greater or less regularity among
stratified rocks (Fig. 34 _b_). They may be regarded as horizontal
dykes, the igneous matter, in place of cutting across the strata,
having forced a way for itself between them. Viewed in this light they
will be found exactly to correspond with ordinary dykes; the rocks on
which they rest, and those which lie above them being both altered like
those on either side of a dyke or vein. A well-known example of this
form of trap is that of Salisbury Crags, where a bed of greenstone
twenty to eighty feet thick is intercalated among sandstones, shales,
and coarse limestones, belonging to the Lower Carboniferous series. Its
under surface presents a remarkably even line, broken at intervals,
however, where the truncated ends of sandstone beds protrude up
into the greenstone, or where the latter cuts into the sandstone
below, occasionally enveloping detached fragments, and sending veins
through them. Along the line of contact both rocks undergo a change.
The greenstone becomes reddened, finer grained, and of a dull earthy
aspect. The sandstones and shales are also red, and excessively hard,
the former resembling a quartz rock, and the latter passing into a sort
of flinty chert or chalcedony. The sandstones above the trap, where
they can be examined, are also found to present the same hardened,
baked appearance, the most intense metamorphism being observable
in those parts which are completely surrounded by igneous matter.
These points were noted many years ago during the famous controversy
between the disciples of Hutton and Werner, the former viewing them
as demonstrative evidence of the igneous origin of the trap-rock, the
latter, on the other hand, professing to see nothing in the section
of the Crags at all militating against the theory that the rocks had
originated from deposition in water. Many a battle was fought in this
locality, and not a few of the trap-dykes and hills possess to the
geologist a classic interest, from having been the examples whence
some of the best established geological opinions were first deduced.
The contest between the Huttonians and Wernerians terminated long ago
in the acknowledged victory of the former; Hutton's doctrines are now
recognised all over the world. It is interesting, however, to walk
over the scenes of the warfare, and mark the very rocks among which it
raged, and from the peculiarities of which it took its rise. Basalts
and greenstones, sandstones and shales, with all their crumplings and
contortions, still stand up as memorials of powerful igneous action,
and of physical changes in the primeval past; and they have become to
the geologist memorials, too, of changes in the onward progress of his
science, where, out of conflicts perhaps yet more tumultuous than those
of ancient Nature, there emerged at last the clear demonstrable truth.
[Illustration: Fig. 34.--Intrusive Trap.]
In the accompanying section (Fig. 34), the more marked characters of
intrusive traps are exhibited. The main mass of igneous rock is the
dyke (_d_), rising through a dislocation or fault, which has thrown
down the beds on one side several feet below those on the other, as
is shown by the interruption of the shale and ironstone beds (_sh_).
The dyke gives off two ramifications, one of them cutting across the
beds obliquely as a vein (_v_); the other passing along the planes
of the shaly layers as a horizontal bed (_b_). The vein, it will be
noticed, produces considerable alteration in its progress, carrying up
and baking a portion of the shale (_sh_), and turning up the edges of
the beds on both sides, which get cracked and hardened along the line
of contact. The bed runs with some regularity for a short distance
through the shales, which show marks of great alteration at their
junction with the trap. Its under surface at one point is seen to have
involved a portion of the shale which has become in consequence highly
metamorphosed, while along the upper surface the bed has sent out a
short irregular vein that twists and otherwise alters the shales
above. These circumstances would suffice to show that even though we
did not find this bed in connexion with a mass of intrusive trap, it
must, nevertheless, have been thrust among previously-formed strata,
and could not have been contemporaneous, that is, poured out along the
sea-bottom before the shales above it were deposited.
But one other form needs to be mentioned here as characteristic of the
Carboniferous intrusive trap-rocks--that of great amorphous masses
which cut through the strata irregularly. They have not the wall-like
form of dykes, nor do they conform to the line of bedding of the rocks
among which they occur. They are sometimes irregular lumps, lying
above or among the strata, and probably connected with some vein or
dyke below. In other localities they look like the upper ends of vast
pillars which may descend into the very depths of Tartarus, as though a
great hole had been blown through the crust of the earth, and a column
of melted matter had risen to fill the cavity. Such masses are often
called _bosses_, and seem not unfrequently to have been the craters
of eruption whence great sheets of lava and showers of ashes were
ejected far and wide over the neighbourhood. They serve to connect
the intrusive traps, whose age is always more or less uncertain, with
the bedded traps properly so called, the geological date of which can
usually be sufficiently ascertained.
II. The bedded or contemporaneous trap-rocks consist of two well-marked
kinds. There are, 1st, the melted rocks, such as greenstones and
basalts and 2d, the tufas and volcanic ashes.
Those of the first-named class differ in no respect from the traps
already noticed, so far as regards mineralogical texture, general
structure, and appearance. In hand specimens the intrusive and bedded
greenstones and basalts cannot be distinguished, nor even when examined
in the field and in masses extending over considerable areas is it
always possible to say to which division any particular hill or crag
should be assigned. The reason of this resemblance is obvious. Where a
trap has either cut through or insinuated itself among rocks of earlier
date it is called intrusive, in relation to the rocks so traversed,
and of course we cannot be sure to what geological period it should be
referred, nor how long an interval may have elapsed between the time
when these rocks were forming and the time when the trap was intruded
among them. If, however, the igneous rock passed upward through these
same strata and then spread out as a flat sheet along the sea-bottom,
the part that came to the top would be termed contemporaneous
with the deposits going on at the time. Hence it follows that all
contemporaneous lava-form trap-rocks are at the same time intrusive as
regards the strata passed through in their progress to the surface. If
the sheet of melted matter that spread out below the water were in the
course of ages worn completely away, along with the strata subsequently
piled above it, so as to leave merely a neck or dyke filling up the
cavity through which the lava rose, we should pronounce the remnant
intrusive, and could form no certain conclusion as to its age or
as to whether its site had ever been a crater actively at work in
throwing out lava and ashes. The sole difference, therefore, between a
contemporaneous and an intrusive greenstone is simply this: the former
rose through a fissure until it reached the surface, and then rolled
out as a flat parallel sheet; the latter may have been erupted from
below at the same time, yet, owing to different circumstances, never
reached the surface, but spread out among or cut through the strata
underneath. And so, when we come to examine in quarries, ravines, and
other exposures, the remains of two such eruptions, we soon ascertain
the relative age of the former from that of the strata among which it
occurs, but as to the date of the latter we are wholly at a loss, for
it gives us no clue by which we can show whether it was erupted before
or after the other. We can but compare the mineralogical character
of the intruded with that of the contemporaneous masses in the same
district, and, from the resemblance which may be traced between them,
draw at the best but a doubtful inference as to their relative dates.
The contemporaneous traps always assume a bedded form, the intrusive
occasionally do so; and the question naturally arises here, what are
the tests whereby a bed of trap may be known to be contemporaneous and
not intrusive? The answer is happily a simple one. An intrusive mass
is found to alter to a greater or less extent the rocks in contact
with it; if it occur as a dyke, then the beds on either side have
been cut through and probably otherwise affected; if it take the form
of a bed or sheet, the strata lying above and below it will be found
to be both altered, showing evidently that a heated mass has been
interposed between them, and consequently that the igneous rock is of
later date than any of the strata among which it occurs. In the case
of a contemporaneous melted trap, however, the appearances presented
are different; it always takes the form of a flat bed corresponding
to all the inclinations and curvatures of the sandstones, shales,
limestones, or other strata among which it lies. If examined carefully,
it may be found not unfrequently baking and contorting the bed that
forms its pavement, but producing no change whatever on that which
composes its roof. It may be capped and underlaid by layers of shale,
and in such a case we might not improbably find the shale below it
highly baked, so as to resemble a sort of rude pottery, while the shale
above would present no sign of such metamorphism, but on the contrary
might display its delicate plants or shells down to the very surface
of the trap, and were the latter concealed from view we should never
suspect, from the aspect of this shale, that any igneous rock existed
in the neighbourhood. The inference to be drawn from such appearances
seems very obvious. Had the upper shale been in existence when the
greenstone or basalt was erupted, it would have suffered an alteration
similar to that produced on the shale below; and the fact, plain and
palpable, that it has undergone no such change, shows pretty clearly
that it was deposited at the bottom of the water after the trap had
cooled and consolidated, and that consequently the trap must be
intermediate in age between the beds on which it rests and those which
lie above it; in other words, that it is a _contemporaneous_ rock.
Hence, if we know the exact geological position and age of the shales,
we know also those of the associated trap, and can thus ascertain
that at a certain definite period in the past history of our planet a
particular district was the scene of volcanic action.
Examples of such contemporaneous traps abound among the carboniferous
rocks of central Scotland, especially in Fife and the Lothians (Fig.
35). I may refer again to the vicinity of Edinburgh as affording some
excellent illustrations. The eastern part of Arthur's Seat displays
a series of basalts and greenstones which can be proved to have been
thrown out during the times of the Lower Carboniferous rocks, at a
period long anterior to that of the Burdiehouse limestone. The Pentland
Hills exhibit on a much greater scale vast sheets of felspathic traps,
such as felstones and tufas, traceable in some cases for six or seven
miles, which were erupted at a still earlier period.[76] The trap
pebbles in our boulder consisted of light yellow and pink felstone,
and were derived, I make no doubt, from these Pentland Hill beds, when
what forms now the cone of Carnethy, rising well-nigh 1900 feet above
the sea, existed as one of a scattered archipelago of islets, or as a
sunken rock battered by the waves that scattered its shingle along the
floor of what may have been either a shallow sea or a shoaling estuary,
where eventually the sand and pebbles hardened into that bed of coarse
grey sandstone whence our boulder was derived.
[Footnote 76: The geology of Arthur's Seat and Pentland Hills was
admirably worked out more than quarter of a century ago by Mr. M'Laren.
His work (already referred to) is unfortunately now out of print.]
The second class of contemporaneous trap-rocks are the tufas or
volcanic ashes. They differ entirely in their aspect and origin from
any of the rocks already described. Greenstones, basalts, felstones,
and such like, were all melted rocks, thrust up from below as we see
lava thrown out by a modern volcano, being styled contemporaneous when
poured out along the sea-bottom or the land, and intrusive when they
never reached the surface but cut through the strata below. The tufas,
however, point to a totally different origin. They are of various
shades of colour, according to their chemical composition. In East
Lothian they assume a deep red hue; among the Pentland Hills they
are often flesh-coloured, while in Linlithgowshire they range from a
dull-brown to a pale leek-green, green being the prevailing tint. They
always show a dull uncrystalline surface, irregularly roughened by
included fragments of various rocks, such as trap, sandstone, shale,
and many others. These fragments or _lapilli_ vary in size from less
than a pin-head up to large bombs of several hundredweight, and from
being generally abundant give to the tufas one of their best-marked
characteristics. The smaller pieces are usually more or less angular,
and throughout the carboniferous series of Linlithgowshire consist
chiefly of a pale felspathic matter, lighter in shade and commonly
harder in texture than the matrix or paste in which they lie. In
some localities, where the included pieces are larger, they have a
rounded form, and often show a honey-combed vesicular surface, like
balls of hardened slag. Fragments of sandstone have not unfrequently
a semi-fused appearance, and plates of shale sometimes look like the
broken debris from a tile-work, although in many instances these
fragments may be found showing no trace whatever of alteration, being
undistinguishable from the neighbouring sandstones and shales from
which they probably came. I have seen in some of the coarser tufas, or
rather volcanic conglomerates, enormous masses of basalt and greenstone
buried deep in the surrounding green or red felspathic paste, and
showing on their more prominent edges the usual vesicular cavities.
In such conglomerates there is usually no division into beds; the
whole mass, indeed, forms a bed between lower and higher strata, but
internally it shows for the most part no trace of stratification. In
these confused assemblages one may occasionally light upon detached
crystals of augite or other mineral scattered irregularly through
the tufa. Their angles will be found often blunted, and the crystals
themselves broken, appearances which have likewise been noticed among
the ash of modern volcanoes. When the tufas are finer grained they
usually exhibit a well-marked stratification, and can often be split
up into laminæ like an ordinary fissile sandstone. Organic remains not
unfrequently abound in such laminated beds, and vary in their character
as widely as in any other stratified rock, being sometimes land-plants,
sometimes sea-shells.
Such are some of the more obvious characters of the volcanic ashes or
tufas, as developed among the carboniferous rocks of central Scotland.
Their great varieties of composition and general aspect render them
a somewhat difficult set of rocks to master, but when fairly and
fully understood they soon prove themselves to be by far the most
interesting section of the traps, for one needs seldom to hesitate a
moment as to their origin or date, while their fossil contents impart
to them an interest all their own. By comparing such rocks with the
consolidated ash or fine dust and _lapilli_ of a modern volcano, a
remarkable resemblance of external characters is found to subsist; and
this likeness holds sufficiently close, when pursued into details, to
show that the ancient and the modern rocks have resulted from the same
source, that, namely, of volcanic eruption. The ash of active burning
mountains falls down their sides loosely and incoherently, every
successive shower of dust or scoriæ settling without much regularity
on those that have gone before. The ash of the old carboniferous
eruptions, however, was showered for the most part over the sea or
across wide shoaling estuaries, at least it is only such portions of
it as fell there that have come down to our day. Settling down among
the mud and sand at the bottom, the volcanic matter accumulated in
wide horizontal beds, every marked inequality being smoothed down by
the currents until a series of regularly stratified layers came to be
formed, entombing any organisms that might find their way to the bottom
or be lying there at the time. The ash of terrestrial volcanoes has no
marked stratification because thrown out in open air, while that of
the carboniferous rocks of central Scotland is distinctly bedded from
having been deposited under water.
Tufas and contemporaneous melted traps are very generally found
together interstratified regularly with each other, and the inference
to be drawn from their juxtaposition is of course simply this, that
at one time liquid lava rolled along the bottom of the water, while
at another showers of volcanic dust and cinders settled down in
successive beds. This active play of the igneous forces took place
at the mouths of estuaries or farther to sea; and it is accordingly
sometimes not a little interesting to trace, amid the sediment that
accumulated below the water during the pauses between the eruptions,
well-preserved remains now of plants that had come drifting from the
land, anon of slim spirifers, and producti that swarmed upon the
hardened lava-streams, and amid the thickening volcanic mud that
slowly sank to the sea-bottom. Such a sequence of events will be made
plain from the following section, the materials of which are derived
from different parts of the trappean region of Linlithgowshire. The
undermost bed here shown (1) is one of marine limestone, abounding
with encrinal joints, corals, spirifers, and other undoubtedly marine
organisms. Above it comes a layer of tufa or volcanic ash (2) of a
dull green aspect, the boundary line between the two rocks lying as
clear as if the quarryman had marked it off with his foot-rule. The
upper part of the ash, however, does not show an equally clear line
of demarcation with the stratum above. On the contrary, it gradually
changes its character, becomes more calcareous as it goes up, with
here and there a stone-lily joint or a stray productus, until these
organisms increase so much in number that the rock insensibly passes
into an ordinary limestone (3) like that below. Next succeeds a thin
seam of ash (4) resting sharply on the limestone and overlaid by a bed
of shale (5) containing the same marine organisms. Another stratum of
ash (6) resembling those below follows the shale, and is surmounted by
a close compact greenstone (7) that hardens the ash on which it rests,
but produces no apparent alteration on the soft fissile shale (8)
above it. Next is a fourth seam of volcanic ash (9) resembling those
below it, but without any shells or crinoidal joints, the only fossils
observable being a few carbonized stems apparently of calamites and
lepidodendra. Above it comes a bed of white quartzy sandstone (10) with
similar vegetable remains, and then a layer of white stiff fire-clay
(11) with rootlets of stigmaria, above which lies a seam of coal (12).
A thin layer of soft blue shale (13) here intervenes, somewhat baked
along its upper portions by another bed of compact vesicular greenstone
(14), which displays in places a well-marked columnar structure. It is
surmounted by a highly characteristic ash (15) in which there occur
numerous large bombs chiefly of trap of different kinds, some of them
highly vesicular. Fragments of shale also occur, mingled here and there
with black carbonized fragments of coal-measure plants, but without any
of the shells and other marine organisms so abundant below. The topmost
bed is a grey carbonaceous sandstone (16), underlying a thin covering
of vegetable mould.
[Illustration: Fig. 35.--Contemporaneous Trap.]
Such is the skeleton, as it were, of the section; the mere dry bones
which remain to the geologist, and which he must study closely to
be able to give them life again. The lowest bed visible, with its
stone-lilies and molluscs, we readily recognise as marking an old
ocean-bed, so that the little episode in the primeval records of
our planet here presented to us opens, like the two great epics of
antiquity, within sound of the wide-roaring sea. The seam of ash which
follows shows, from the sharpness of its line of demarcation with the
limestone, how the denizens of the sea-bottom were suddenly destroyed
by a thick shower of volcanic dust that settled down over their
remains. The waters, however, soon cleared, and ere long stone-lilies
and producti were plentiful as ever, mingling their remains among the
upper layers of the soft muddy ash, and giving rise therefrom to a
sort of calcareous ash or ashy limestone, until in the course of time
the volcanic matter became wholly covered over by a seam of ordinary
limestone. The corals and stone-lilies were, however, anew destroyed
by the deposition of volcanic dust that settled over them as a seam
of ash, after which the water was again rendered turbid and muddy by
the inroad of foreign matter, which, brought down by rivers or by the
changing currents of the ocean, sank to the bottom and eventually
consolidated into a seam of shale. Thereafter the volcanic forces
began once more to eject a quantity of dust and scoriæ that fell into
the water and spread along the bottom as a stratum of ash, and to
pour out a current of lava which hardened into a great sheet visible
now as the undermost greenstone of the section. The emission of the
lava seems to have terminated the eruption, for the next stratum is
one of shale like that below the ash, so that the muddy sediment,
the deposition of which was interrupted for a while by the volcanic
products, began afresh to settle down along the sea-bottom. This
last condition of things seems to have continued for a considerable
period, seeing that the shale bed is relatively thick, and from its
fissile laminated structure indicates a slow and tranquil deposition.
Another eruption of volcanic dust and ashes again interrupted the
detrital deposits, and gave rise to another seam of tufa. This last
subterranean movement seems to have considerably altered the general
contour of the sea-bottom, and so elevated it, at least at one part,
that a thick accumulation of sand, and subsequently of clay, filled
it up to the level of the water or nearly so, giving rise to a dense
growth of the stigmaria and other coal-measure plants whose roots are
still seen imbedded in the fire-clay on which, as a soft muddy soil,
they originally grew. It is probable, however, that, notwithstanding
such elevations of the sea-bed, there was a general subsidence of the
ground during the accumulation of these strata, for we see that the
peaty morass, represented now by the coal-seam, ere long sank beneath
the waters, with the inroads of which it was unable to keep pace, while
there slowly silted over it a muddy sediment that hardened at length
into what is now a seam of shale. But this order of things had been in
existence for but a comparatively short period when the igneous forces
broke out again, ejecting a stream, of molten lava that spread along
the bottom of the shallow waters and hardened as before into a sheet
of greenstone. This was followed by an abundant shower of dust and
lapilli, along with numerous large masses of greenstone and basalt.
These falling into the water accumulated on the upper surface of the
lava-stream, then somewhat cooled, and formed in the end a stratum of
ash of a rubbly conglomeritic aspect. That the sheet of greenstone
really spread out along the sea-bottom before the ejection of the ash,
and was not intruded among the beds at a later period,--that, in short,
it must be regarded as a contemporaneous and not as an intrusive rock,
seems sufficiently shown by its great regularity and evenness, and by
the unaltered condition of the fine soft felspathic matter which covers
its upper surface. It was assuredly in a highly-heated condition when
poured out, as may be gathered from the baked aspect of the mud over
which it rolled; but it had cooled and solidified, at least along its
upper surface, ere buried beneath the shower of ashes. The last bed
exhibited in the section is a grey sandstone, with many carbonaceous
streaks and traces of land-plants, showing a pause in the volcanic
activity of the district, during which the streams from the land
brought down sandy sediment, with an abundant admixture of macerated
leaves, branches, and other drift-wood.
It thus appears that not only were the plains and hills of the
Carboniferous era richly clothed with vegetation, and its waters
crowded with animals, but that then, as now, subterranean forces were
at work, sometimes elevating, sometimes depressing the area alike of
the land and of the sea; while, not unfrequently, melted lava rose
from below, rolling along the bottom of the waters, and showers of
ashes were flung far and wide through the air, settling at last as a
thickening sediment along the floor of the sea, or athwart the marshy
swamps of the delta. Whether the interior of the land had burning cones
among its pine-covered hills we know not yet. Such, however, probably
existed; nay, there may have been among the higher peaks some "snowy
pillar of heaven," like the Ætna of Pindar, raising its smoking summit
among everlasting crags of ice in solitudes lifeless and bare.[77]
[Footnote 77: The highest points of New Zealand, nearly 10,000 feet
above the sea, are said to be clothed for two-thirds of their height
with ice and snow. If, therefore, during Carboniferous times, there
existed somewhere to the west of what is now central Scotland, a chain
cf hills 5000 or 6000 feet high, their summits might perhaps have been
as wintry us that of Mont Blanc.]
Our boulder has served us like the minstrels in modern Gothic
poetry, who appear between the cantos, and give an air of unity and
completeness to what would otherwise be often rambling and unconnected.
And now, at the close, it comes again before us, lying in its bed of
clay, clustered with mosses of brightest green, and overshadowed by its
flickering canopy of beechen leaves. Silent and senseless, the emblem,
seemingly, of calm repose and unchanging durability, what could we have
conceived it should have to chronicle, save the passing, perchance, of
many a wintry December and many a sultry June. Such, indeed, would be
the character of its records of the centuries that have passed away
since the birth of man, did any such record survive in its keeping.
But it rests there as the memorial of far earlier centuries, and of
an older creation; and though now surrounded with all that is lovely
or picturesque--the twinkling flowers on every side, the wide arch of
boughs overhead, and the murmuring streamlet in the dell below--and
though forming itself no unimpressive object in the scene, the boulder
looks out upon us unconnected with anything around. Like a sculptured
obelisk transported from the plains of Assyria to the streets of
London, it offers no link of association with the order of things
around it; its inscriptions are written in hieroglyphics long since
extinct, but of which the key yet remains to show us that the rocks of
our planet are not masses of dead, shapeless matter, but chronicles of
the past; and that all the varied beauty of green field and waving wood
is but a thin veil of gossamer spread out over the countless monuments
of the dead. We have raised one little corner of this gauze-like
covering, and tried to decipher the memorials of bygone creations,
traced in clear and legible characters on the boulder. First, there
lies spread out before us a wide arctic sea, studded with icebergs
that come drifting from the north. Here and there a bare barren islet
rises above the waste of waters, and the packed ice-floes often strand
along its shores, while at other parts great towering bergs, aground
in mid-ocean, keep rising and falling with the heavings of the surge,
and seem ever on the verge of toppling into the deep. But this scene,
so bleak and lifeless, erelong fades away, and we can descry a wide
archipelago of islands, green well-nigh to the water's edge, and
looking like the higher hill-tops of some foundered continent. The
waves are actively at work wearing down the shores, which present for
the most part an abrupt cliff-line to the west. This picture, too,
gets gradually dim, and when the darkness and haze have cleared away,
the scene is wholly new. For miles around there spreads out an expanse
of water, like a wide lake, thickly dotted with islets of every form
and size, clothed with a rich vegetation. Here a jungle of tall reeds
shoots out of the water, clustering with star-like leaves; there a
group of graceful trees, fluted like the columns of an ancient temple,
and crowned by a coronal of sweeping fronds, spread out their roots
amid the soft mud. Yonder lies a drier islet, rolling with ferns of
every shape and size, with here and there a lofty tree-fern, waving its
massive boughs high overhead. The vegetation, rank and luxuriant in the
extreme, strikes us as different from anything visible at the present
day, though, as our eyes rest on the muddy discoloured current, we
can mark, now and then, huge trunks, branchless and bare, that recall
some of the living pine-trees. The denizens of the water seem to be
equally strange. Occasionally a massive head, with sharp formidable
tusks, peers above the surface, and then the gleam of fins and scales
reveals a creature some twenty or thirty feet long. Glancing down into
the clearer spots, we can detect many other forms of the finny tribes,
all cased in a strong glistening armature of scales, and darting
about with ceaseless activity. Beyond this scene of almost tropical
luxuriance, on the one side, lies the blue ocean, with its countless
shells and corals, its stone-lilies and sea-urchins, and its large
predaceous fish; on the other side stretches a far-off chain of hills,
whose nether slopes, dark with pine-woods, sweep down into the rich
alluvial plains. And then this landscape, too, fades slowly away, and
thick darkness descends upon us. Yet through the gloom we feel ever
and anon the rambling earthquake, and see in the distance the glare
of some active volcano that throws a ruddy gleam amid the pumice and
ashes, ever dancing along the surface of the sea. And now this last
scene melts away like the rest, and dark night comes down in which we
can detect no ray of light, and beyond which we cannot go. The record
of the boulder can conduct us no further into the history of the past.
The same principles which have been pursued in the previous pages in
elucidating the history of the Carboniferous system, will conduct
the reader to the true origin and age of any group of rocks he may
encounter, whatever its nature, and wheresoever its locality. Let
him, therefore, in his country rambles, seek to verify them in valley
and hill-side, by lake and cataract, and along river-course and
sea-shore. Let him not be content with simply admiring the picturesque
grouping of rock-masses, but rather seek to interpret their origin
and history, tracing them step by step into the past, amid ages long
prior to man. Such a process will give him a yet keener relish for
the beauties of their scenery, by ever calling up to his mind some of
those striking contrasts with which geology abounds. In the stillness
of the mountain-glen, he will see on every side traces of the waves of
ocean, and when dipping his oars into the unruffled sea among groups
of wasted rocks, miles from shore, he will bethink him, perchance,
of some old forest-covered land, of which these battered islets are
the sole memorials. His enjoyment of the scenery of nature is thus
increased manifold, and he carries about with him a power of making
even the tamest landscape interesting. Cowper, in one of his exquisite
letters, remarks,--"Everything I see in the fields, is to me an object;
and I can look at the same rivulet or at a handsome tree, every day of
my life, with new pleasure." Had the sweet singer of Olney lived to
witness the results obtained by the geologists whom he satirized, he
would perhaps have sauntered along the Ouse with a new pleasure, and
have felt a yet more intense delight in casting his eyes athwart the
breadth of landscape that spreads out around-the "Peasant's nest."
Such, however, are after all only secondary incentives to the study of
the rocks. As a mental exercise, geology certainly yields to none of
the other sciences, for it addresses itself at once to the reasoning
powers and to the imagination, and may thus be made a source both
of intellectual training and of delightful recreation. Of none of
the sciences is it so easy to get a general smatter, yet none is so
difficult thoroughly to master, for geology embraces all the sciences.
In so wide a field, the student will therefore find ample room to
expatiate. In beginning the study, he may perhaps think it, as Milton
pictured the other paths of learning, "laborious, indeed, at the first
ascent; but else so smooth, so green, so full of goodly prospects and
melodious sounds on every side, that the harp of Orpheus was not more
charming." If time and taste disincline him to travel over the whole
of the broad field, there are delightful nooks to which he may betake
himself, replete with objects of beauty and interest, where he may
spend his leisure, and by so doing not merely delight himself, but
enlarge the bounds of human knowledge. No part of the domain can be too
obscure or remote to reward his attention; no object too trifling or
insignificant: for the march of science, though a stately one, proceeds
not by strides, but by steps often toilsome and slow; and she stands
mainly indebted for her progress not to the genius of a few gigantic
intellects, but to the united efforts of many hundred labourers, each
working quietly in his own limited sphere.
But the highest inducement to this study must ever be that so quaintly
put by old Sir Thomas Browne: "The world was made to be inhabited by
beasts, but studied and contemplated by man: 'tis the debt of our
reason we owe unto God, and the homage we pay for not being beasts;
without this the world is still as though it had not been, or as it
was before the sixth day, when as yet there was not a creature that
could conceive or say there was a world." Geology lifts off for us the
veil that shrouds the past, and lays bare the monuments of successive
creations that had come and gone long ere the human race began. She
traces out the plan of the Divine working during a vast cycle of ages-,
and points out how the past dovetails with the present, and how the
existing condition of things comes in as but the last and archetypal
economy in a long progressive series. By thus revealing what has gone
before, she enables us more fully to understand what we see around us
now. Much that is incomplete she restores; much that is enigmatical
she explains. She teaches us more fully man's true position in the
created universe, by showing that in him all the geologic ages meet
that he is the point towards which creation has ever been tending. How
far the facts brought to light by geology may bear upon the future,
will not, perhaps, be solved until that future shall have come. There
is, nevertheless, in the meanwhile, material enough for solemn and
earnest reflection, and as years go by the amount will probably be
always increasing. For we must ever be only learners here, and when all
earthly titles and distinctions have passed away, and we enter amid the
realities of another world, we shall carry with us this one common name
alone. It will, perhaps, be then as now, that only
"In contemplation of created things
By steps we may ascend to God."
And it can surely be no unmeet preparation for such a scene, in humble
faith to read the records of His doings which the Almighty has graven
on the rocks around us. Many problems meet us on every hand problems
which it seems impossible for us now to solve and as the circle of
science ever widens, its enveloping circumference of difficulty and
darkness widens in proportion. It is, doubtless, well that it should be
so; for we are thus taught to regard our present state as imperfect and
incomplete, and to long for that higher and happier one promised by the
Redeemer to those that love Him, when "we shall know thoroughly even as
we are thoroughly known."
TABLE OF FOSSILIFEROUS ROCKS.
LYELL'S _Elements_, p. 109.
1. RECENT. } POST-TERTIARY.
2. POST-PLIOCENE. }
}
3. NEWER PLIOCENE. } PLIOCENE. }
4. OLDER PLIOCENE. } }
} TERTIARY
5. UPPER MIOCENE. } MIOCENE. } or
6. LOWER MIOCENE. } } CAINOZOIC.
}
7. _a_ UPPER EOCENE. } EOCENE. }
7. _b_ MIDDLE EOCENE. } }
8. LOWER EOCENE. } }
9. MAESTRICHT BEDS. }
10. UPPER WHITE CHALK. }
11. LOWER WHITE CHALK. }
12. UPPER GREENSAND. } CRETACEOUS. }
13. GAULT. } }
14. LOWER GREENSAND. } }
15. WEALDEN. } }
}
16. PURBECK BEDS. } }
17. PORTLAND STONE. } } SECONDARY
18. KIMMERIDGE CLAY. } } or
19. CORAL RAG. } JURASSIC } MESOZOIC.
20. OXFORD CLAY. } }
21. GREAT or BATH OOLITE. } }
22. INFERIOR OOLITE. } }
23. LIAS. } }
}
24. UPPER TRIAS. } }
25. MIDDLE TRIAS, } }
or } TRIASSIC. }
MUSCHELKALK. } }
26. LOWER TRIAS. } }
27. PERMIAN, }
or } PERMIAN. }
MAGNESIAN LIMESTONE. } }
}
28. COAL-MEASURES. } }
29. CARBONIFEROUS } CARBONIFEROUS. }
LIMESTONE. } }
} PRIMARY
30. UPPER DEVONIAN. } DEVONIAN, or } or
31. LOWER " } OLD RED SANDSTONE. } PALÆOZOIC.
}
32. UPPER SILURIAN. } SILURIAN. }
33. LOWER " } }
}
34. UPPER CAMBRIAN. } CAMBRIAN. }
35. LOWER " } }
* * * * *
Transcriber Note
Minor typos corrected. Some images moved to nearest paragraph break. A
paragraph break was added to page 74 and 105 to accommodate placement
of Figures 17 and 26 respectively. The missing anchor for the footnote
on page 199 was added.
*** End of this LibraryBlog Digital Book "The Story of a Boulder; or, Gleanings from the Note-book of a Field Geologist" ***
Copyright 2023 LibraryBlog. All rights reserved.